Imaging device and electronic device

ABSTRACT

An imaging device which has a stacked-layer structure and can be manufactured easily is provided. The imaging device includes a signal processing circuit, a memory device, and an image sensor. The imaging device has a stacked-layer structure in which the memory device is provided above the signal processing circuit, and the image sensor is provided above the memory device. The signal processing circuit includes a transistor formed on a first semiconductor substrate, the memory device includes a transistor including a metal oxide in a channel formation region, and the image sensor includes a transistor formed on a second semiconductor substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/763,024, filed May 11, 2020, now allowed, which is incorporated byreference and is a U.S. National Phase Application under 35 U.S.C. § 371of International Application PCT/IB2018/058883, filed on Nov. 13, 2018,which is incorporated by reference and claims the benefit of foreignpriority applications filed in Japan on Nov. 23, 2017, as ApplicationNo. 2017-225285, and on Feb. 23, 2018, as Application No. 2018-030881.

TECHNICAL FIELD

One embodiment of the present invention relates to an imaging device. Inparticular, one embodiment of the present invention relates to animaging device with a stacked-layer structure.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of the invention disclosed inthis specification and the like relates to an object, a method, or amanufacturing method. One embodiment of the present invention relates toa process, a machine, manufacture, or a composition of matter.

BACKGROUND ART

A MOS image sensor typified by CMOS (Complementary Metal OxideSemiconductor) is known as an imaging device that is widely used forelectronic devices, such as a digital video camera, a digital camera,and a smartphone.

In recent years, a stacked-layer structure in which a plurality ofsemiconductor chips having different functions are stacked to beelectrically connected has been in practical use for an imaging device.By employing the stacked-layer structure, manufacturing methods andmaterials used for the semiconductor chips can be selected to match therespective functions of the semiconductor chips; accordingly, a moresophisticated imaging device can be easily achieved and the size andweight of an electronic device including the imaging device can bereduced.

Non-Patent Document 1 discloses an example of an imaging device in whicha signal processing circuit, a memory device, and a MOS image sensor arestacked. Non-Patent Document 1 describes that, with the structure inwhich a DRAM (Dynamic Random Access Memory) is used as a memory deviceand data of a captured image is stored in the DRAM, high-speed imagingexceeding a speed for transferring data to and from an electronic deviceincluding the imaging device can be achieved.

Meanwhile, a silicon-based semiconductor material is widely known as asemiconductor that can be used in a transistor, and as another material,an oxide semiconductor has attracted attention. As the oxidesemiconductor, not only single-component metal oxides, such as indiumoxide and zinc oxide, but also multi-component metal oxides are known.Among the multi-component metal oxides, in particular, an In—Ga—Zn oxide(hereinafter also referred to as an IGZO) has been actively studied.

From the studies on IGZO, a CAAC (c-axis aligned crystalline) structureand an nc (nanocrystalline) structure, which are not single crystal noramorphous, have been found in an oxide semiconductor (see Non-PatentDocument 2 to Non-Patent Document 4). In Non-Patent Document 2 andNon-Patent Document 3, a technique for manufacturing a transistor usingan oxide semiconductor having a CAAC structure is also disclosed.Moreover, Non-Patent Document 5 and Non-Patent Document 6 show that afine crystal is included even in an oxide semiconductor which has lowercrystallinity than an oxide semiconductor having the CAAC structure orthe nc structure.

In addition, a transistor using an oxide semiconductor has an extremelylow off-state current (see Non-Patent Document 7), and an LSI and adisplay utilizing the characteristics have been reported (see Non-PatentDocument 8 and Non-Patent Document 9).

A DRAM in which a transistor using an oxide semiconductor is used for aDRAM cell is suggested (e.g., Patent Document 1 and Non-Patent Document10). The transistor using an oxide semiconductor has an extremely lowleakage current in an off state (off-state current), and thus enablesfabrication of a low-power-consumption memory having long refreshintervals. In this specification and the like, a DRAM in which thetransistor using an oxide semiconductor is used for a DRAM cell isreferred to as an “oxide semiconductor DRAM” or a “DOSRAM (registeredtrademark, Dynamic Oxide Semiconductor Random Access Memory)”.

REFERENCE

[Patent Document]

-   [Patent Document 1] Japanese Published Patent Application No.    2012-256820

Non-Patent Document

-   [Non-patent Document 1] Tsutomu Haruta et al., “A 1/2.3 inch 20M    pixel 3-Layer Stacked CMOS Image Sensor with DRAM”, IEEE ISSCC Dig.    Tech. Papers, 2017, pp. 76-78.-   [Non-Patent Document 2] S. Yamazaki et al., “SID Symposium Digest of    Technical Papers”, 2012, volume 43, issue 1, pp. 183-186.-   [Non-Patent Document 3] S. Yamazaki et al., “Japanese Journal of    Applied Physics”, 2014, volume 53, Number 4S, pp.    04ED18-1-04ED18-10.-   [Non-Patent Document 4] S. Ito et al., “The Proceedings of AM-FPD′13    Digest of Technical Papers”, 2013, pp. 151-154.-   [Non-Patent Document 5] S. Yamazaki et al., “ECS Journal of Solid    State Science and Technology”, 2014, volume 3, issue 9,    pp.Q3012—Q3022.-   [Non-Patent Document 6] S. Yamazaki, “ECS Transactions”, 2014,    volume 64, issue 10, pp. 155-164.-   [Non-Patent Document 7] K. Kato et al., “Japanese Journal of Applied    Physics”, 2012, volume 51, pp. 021201-1-021201-7.-   [Non-Patent Document 8] S. Matsuda et al., “2015 Symposium on VLSI    Technology Digest of Technical Papers”, 2015, pp.T216—T217.-   [Non-Patent Document 9] S. Amano et al., “SID Symposium Digest of    Technical Papers”, 2010, volume 41, issue 1, pp. 626-629.-   [Non-Patent Document 10] T. Onuki, et al.,“DRAM with Storage    Capacitance of 3.9 fF using CAAC-OS Transistor with L of 60 nm and    having More Than 1-h Retention Characteristics,” Ext. Abstr. SSDM,    2014, pp. 430-431.

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In Non-Patent Document 1, a signal processing circuit, a memory device,and a MOS image sensor are formed over different semiconductor chips,and the three semiconductor chips are stacked to be electricallyconnected to one another. Here, when surfaces where the signalprocessing circuit, the memory device, and the MOS image sensor areformed are called top surfaces of the semiconductor chips, specifically,the top surface of the semiconductor chip in the second layer (thememory device) is attached to the top surface of the semiconductor chipin the first layer (the signal processing circuit) such that the topsurfaces face each other, and then the semiconductor chip in the secondlayer is thinned. Moreover, the top surface of the semiconductor chip inthe third layer (the MOS image sensor) is attached to the semiconductorchip in the second layer, and then the semiconductor chip in the thirdlayer is thinned. In this manner, an imaging device including a “backilluminated” MOS image sensor is manufactured in which light that entersfrom a bottom surface (thinned surface) of the semiconductor chip in thethird layer is detected.

However, the above manufacturing method is difficult because the methodhas two attaching steps and two thinning steps, and moreover, a siliconsubstrate of the semiconductor chip in the second layer needs to have apenetrating portion for electrical connection. Accordingly, it is noteasy to increase the storage capacity by an increase in the number ofstacked memory devices. There is also a problem in that attachment ofthe semiconductor chips has a problem of heat generation but the DRAMemployed as the memory device is relatively not resistant to heat.

An object of one embodiment of the present invention is to provide animaging device which has a stacked-layer structure and can be easilymanufactured. Another object is to provide an imaging device which has astacked-layer structure and whose storage capacity can be easilyincreased. Another object is to provide an imaging device which has astacked-layer structure and includes a highly reliable memory devicewith high heat resistance.

Another object of one embodiment of the present invention is to providea novel imaging device. Another object of one embodiment of the presentinvention is to provide an electronic device including a novel imagingdevice.

Note that one embodiment of the present invention does not necessarilyachieve all the objects listed above and only needs to achieve at leastone of the objects. The description of the above objects does notpreclude the existence of other objects. Objects other than these willbe apparent from the description of the specification, the claims, thedrawings, and the like, and objects other than these can be derived fromthe description of the specification, the claims, the drawings, and thelike.

Means for Solving the Problems

One embodiment of the present invention is an imaging device including afirst layer, a second layer above the first layer, and a third layerabove the second layer. The first layer includes a signal processingcircuit, the second layer includes a memory device, and the third layerincludes an image sensor. The signal processing circuit includes atransistor formed on a first semiconductor substrate, the memory deviceincludes a transistor including a metal oxide in a channel formationregion, and the image sensor includes a transistor formed on a secondsemiconductor substrate.

Another embodiment of the present invention is an imaging deviceincluding a first layer, a second layer above the first layer, and athird layer above the second layer. The first layer includes a signalprocessing circuit, the second layer includes a memory device, and thethird layer includes an image sensor. The memory device includes aplurality of memory cells, the memory cell includes a transistor and acapacitor, one of a source and a drain of the transistor is electricallyconnected to one electrode of the capacitor, and the transistor includesa metal oxide in a channel formation region. The signal processingcircuit includes a transistor formed on a first semiconductor substrate,and the image sensor includes a transistor formed on a secondsemiconductor substrate.

In the above embodiment, the signal processing circuit may have afunction of controlling operation of the memory device and the imagesensor.

In the above embodiment, at least one of the signal processing circuitand the image sensor may include an analog-digital conversion circuit,and the analog-digital conversion circuit may have a function ofconverting image data generated by the image sensor to a digital signal.

In the above embodiment, the memory device may have a function ofretaining the digital signal.

Effect of the Invention

One embodiment of the present invention can provide an imaging devicewhich has a stacked-layer structure and can be easily manufactured. Animaging device which has a stacked-layer structure and whose storagecapacity can be easily increased can be provided. An imaging devicewhich has a stacked-layer structure and includes a highly reliablememory device with high heat resistance can be provided.

An imaging device which has a stacked-layer structure and the cost ofwhich is lowered can be provided. An imaging device which has astacked-layer structure and whose power consumption is reduced by areduction in the refresh rate can be provided.

One embodiment of the present invention can provide a novel imagingdevice. One embodiment of the present invention can provide anelectronic device including a novel imaging device.

Note that the description of the effects does not disturb the existenceof other effects. One embodiment of the present invention does notnecessarily have all the effects. Effects other than these will beapparent from the description of the specification, the claims, thedrawings, and the like, and effects other than these can be derived fromthe description of the specification, the claims, the drawings, and thelike.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are perspective views illustrating a structure exampleof an imaging device.

FIGS. 2A-2C are circuit diagrams illustrating configuration examples ofa memory cell, and FIG. 2D is a circuit diagram illustrating aconfiguration example of a pixel.

FIG. 3 is a perspective view illustrating a structure example of animaging device.

FIG. 4 is a cross-sectional view illustrating a structure example of alayer 100 and a layer 200.

FIGS. 5A-5C are cross-sectional views illustrating structure examples oftransistors.

FIG. 6 is a cross-sectional view illustrating a structure example of animaging device.

FIG. 7 is a cross-sectional view illustrating a structure example of animaging device.

FIGS. 8A and 8B are cross-sectional views illustrating a structureexample of a transistor.

FIG. 9A is a top view illustrating a structure example of a transistor,FIG. 9B is a perspective view illustrating the structure example of thetransistor.

FIGS. 10A and 10B are cross-sectional views illustrating a structureexample of a transistor.

FIGS. 11A and 11C are cross-sectional views of transistors, and FIGS.11B and 11D are electrical characteristics of the transistors.

FIGS. 12A-12D are views illustrating structure examples of electronicdevices.

FIG. 13 is a view illustrating a structure example of an electronicdevice.

FIGS. 14A and 14B are graphs showing Shmoo plots.

FIG. 15 is a graph showing memory retention characteristics.

FIGS. 16A-16C are graphs showing estimated power consumption.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments will be described with reference to drawings.However, the embodiments can be implemented with many different modes,and it will be readily appreciated by those skilled in the art thatmodes and details thereof can be changed in various ways withoutdeparting from the spirit and scope thereof. Thus, the present inventionshould not be construed as being limited to the description in theembodiments given below.

A plurality of embodiments described below can be combined asappropriate. In addition, in the case where a plurality of structureexamples are described in one embodiment, the structure examples can becombined as appropriate.

In the drawings and the like, the size, the layer thickness, the region,or the like is exaggerated for clarity in some cases. Therefore, thesize, the layer thickness, or the region is not limited to theillustrated scale. The drawings schematically show ideal examples, andshapes, values, or the like are not limited to shapes, values, or thelike shown in the drawings.

In the drawings and the like, the same elements, elements having similarfunctions, elements formed of the same material, elements formed at thesame time, or the like are sometimes denoted by the same referencenumerals, and description thereof is not repeated in some cases.

In this specification and the like, the term “film” and the term “layer”can be interchanged with each other. For example, the term “conductivelayer” can be changed into the term “conductive film” in some cases. Foranother example, the term “insulating film” can be changed into the term“insulating layer” in some cases.

In this specification and the like, the terms for describing arrangementsuch as “over” and “below” do not necessarily mean “directly over” and“directly below”, respectively, in the positional relationship betweencomponents. For example, the expression “a gate electrode over a gateinsulating layer” does not exclude the case where there is an additionalcomponent between the gate insulating layer and the gate electrode.

In this specification and the like, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

In this specification and the like, “electrically connected” includesthe case where connection is made through an “object having any electricfunction”. Here, there is no particular limitation on the “object havingany electric function” as long as electric signals can be transmittedand received between the connected components. Examples of the “objecthaving any electric function” include a switching element such as atransistor, a resistor, an inductor, a capacitor, and other elementswith a variety of functions as well as an electrode and a wiring.

In this specification and the like, “voltage” often refers to apotential difference between a given potential and a reference potential(e.g., a ground potential). Thus, a voltage and a potential differencecan be interchanged with each other.

In this specification and the like, a transistor is an element having atleast three terminals including a gate, a drain, and a source. A channelformation region is included between the drain (a drain terminal, adrain region, or a drain electrode) and the source (a source terminal, asource region, or a source electrode), and current can flow between thesource and the drain through the channel formation region. Note that inthis specification and the like, a channel formation region refers to aregion through which current mainly flows.

Furthermore, functions of a source and a drain might be switched when atransistor of opposite polarity is employed or a direction of currentflow is changed in circuit operation, for example. Thus, the terms ofsource and drain are interchangeable for use in this specification andthe like.

Unless otherwise specified, off-state current in this specification andthe like refers to a drain current of a transistor in an off state (alsoreferred to as a non-conducting state or a cutoff state). Unlessotherwise specified, the off state of an n-channel transistor refers toa state where a voltage Vgs of a gate with respect to a source is lowerthan a threshold voltage Vth, and the off state of a p-channeltransistor refers to a state where the voltage Vgs of a gate withrespect to a source is higher than the threshold voltage Vth. That is,the off-state current of an n-channel transistor sometimes refers to adrain current at the time when the voltage Vgs of a gate with respect toa source is lower than the threshold voltage Vth.

In the above description of the off-state current, the drain may bereplaced with the source. That is, the off-state current sometimesrefers to a source current when the transistor is in an off state. Inaddition, leakage current sometimes expresses the same meaning asoff-state current. In this specification and the like, the off-statecurrent sometimes refers to a current that flows between a source and adrain when a transistor is in the off state.

In this specification and the like, a metal oxide means an oxide ofmetal in a broad sense. Metal oxides are classified into an oxideinsulator, an oxide conductor (including a transparent oxide conductor),an oxide semiconductor, and the like. For example, in the case where ametal oxide is used in a channel formation region of a transistor, themetal oxide is called an oxide semiconductor in some cases. That is, inthe case where a metal oxide has at least one of an amplifying function,a rectifying function, and a switching function, the metal oxide can becalled a metal oxide semiconductor. Hereinafter, a transistor includinga metal oxide in a channel formation region is also referred to as an“oxide semiconductor transistor” or an “OS transistor”. Furthermore, the“transistor including an oxide semiconductor” described above is alsothe transistor including a metal oxide in a channel formation region.

Furthermore, in this specification and the like, a metal oxidecontaining nitrogen is in some cases also collectively referred to as ametal oxide. A metal oxide containing nitrogen may be referred to as ametal oxynitride. The details of a metal oxide will be described later.

Embodiment 1

In this embodiment, structure examples of an imaging device ofembodiments of the present invention will be described. An imagingdevice of one embodiment of the present invention has a structure inwhich a layer including a transistor formed over a semiconductorsubstrate and a layer including an OS transistor are stacked, and thetransistor formed over the semiconductor substrate and a layer includinga photodiode are stacked and bonded.

Structure Example of Imaging Device

FIG. 1(A) is a perspective view illustrating a structure example of animaging device 10 of one embodiment of the present invention.

The imaging device 10 includes a layer 100, a layer 200, and a layer300. The imaging device 10 has a stacked-layer structure in which thelayer 200 is stacked over the layer 100, and the layer 300 is stackedover the layer 200 as illustrated in FIG. 1(A). Note that an interlayerinsulating layer can be provided between the layer 100 and the layer200.

FIG. 1(B) is a perspective view for easy understanding of the structureexample of the imaging device 10.

Each of the layer 100, the layer 200, and the layer 300 is provided witha device or a circuit which can function by utilizing semiconductorcharacteristics; the layer 100 is provided with a signal processingcircuit 110, the layer 200 is provided with a memory device 210, and thelayer 300 is provided with an image sensor 310.

<Signal Processing Circuit 110>

The signal processing circuit 110 has a function of controllingoperation of the memory device 210 and the image sensor 310, a functionof processing image data generated by the image sensor 310, and afunction of transmitting and receiving data, a control signal, and thelike to and from an electronic device including the imaging device 10,for example.

Specifically, for example, the signal processing circuit 110 includes acontrol circuit 111, a control circuit 112, an image processing circuit113, and an input/output circuit 114 (see FIG. 1(B)).

The control circuit 111 has a function of supplying data to be writtento the memory device 210, an address signal for specifying an address ofthe memory device 210 that performs data reading/writing, a controlsignal for controlling operation of the memory device 210, and the like.The control circuit 111 also has a function of receiving data read outfrom the memory device 210.

The control circuit 112 has a function of receiving image data generatedby the image sensor 310 and a function of supplying a control signal orthe like for controlling the operation of the image sensor 310. Thecontrol circuit 112 may include an analog-digital conversion circuit(Analog-Digital Converter: ADC).

The image processing circuit 113 has a function of performing, forexample, gamma correction, dimming, toning, noise removal, distortioncorrection, video codic, and the like on image data generated by theimage sensor 310. The image processing circuit 113 may also have afunction of performing face detection, automatic scene recognition, andhigh dynamic range rendering (HDR).

Here, the automatic scene recognition means that a scene such as ambientenvironment is recognized and exposure, focus, flash, and the like areautomatically adjusted. The image processing circuit 113 does not needto perform all of the above-described processing steps and can selectprocessing steps as needed.

The input/output circuit 114 has a function of transmitting andreceiving data, a control signal, and the like to/from the electronicdevice including the imaging device 10. For the input/output circuit114, for example, an interface, such as LVDS (Low-Voltage DifferentialSignaling), MIPI (Mobile Industry Processor Interface), or SPI (SerialPeripheral Interface), is used.

In addition, the signal processing circuit 110 may include a bus line115, a power supply circuit 116, and the like. FIG. 1(B) illustrates astate in which the control circuit 111, the control circuit 112, theimage processing circuit 113, and the input/output circuit 114 areconnected through the bus line 115.

The signal processing circuit 110 is formed with transistors formed overa semiconductor substrate SUB1. The semiconductor substrate SUB1 is notparticularly limited as long as a channel region of a transistor can beformed thereon. For example, a single crystal silicon substrate, asingle crystal germanium substrate, a compound semiconductor substrate(such as a SiC substrate or a GaN substrate), an SOI (Silicon onInsulator) substrate, or the like can be used.

As the SOI substrate, the following substrate may be used: an SIMOX(Separation by Implanted Oxygen) substrate which is formed in such amanner that after an oxygen ion is implanted into a mirror-polishedwafer, an oxide layer is formed at a certain depth from the surface anddefects generated in a surface layer are eliminated by high-temperatureannealing, or an SOI substrate formed by using a Smart-Cut method inwhich a semiconductor substrate is cleaved by utilizing growth of aminute void, which is formed by implantation of a hydrogen ion, bythermal treatment; an ELTRAN method (a registered trademark: EpitaxialLayer Transfer). A transistor formed using a single crystal substratecontains a single crystal semiconductor in a channel region.

In this embodiment, a case in which a single crystal silicon substrateis used as the semiconductor substrate SUB1 will be described as anexample. Hereinafter, a transistor formed over a single crystal siliconsubstrate is referred to as a Si transistor.

<Memory Device 210>

The memory device 210 is connected to the control circuit 111 through awiring CL that connects the layer 100 to the layer 200. The wiring CL isformed of a conductor formed in a contact hole in the layer 100 and thelayer 200. Then, input/output of data and signals between the memorydevice 210 and the control circuit 111 are performed through the wiringCL.

The memory device 210 provided in the layer 200 includes a cell array211, a driver circuit 221, and a driver circuit 222. The cell array 211is composed of a plurality of memory cells 212 arranged in a matrix.

The memory cell 212 has a function of storing data. The memory cell 212may have a function of storing binary (high level and low level) data ormay have a function of storing multilevel data of four or more levels.The memory cell 212 may have a function of storing analog data.

The driver circuit 221 has a function of selecting the memory cell 212.Specifically, the driver circuit 221 has a function of supplying asignal for selecting a memory cell 212 which is subjected to datawriting or reading (hereinafter also referred to as a selection signal)to a wiring connected to the memory cell 212.

The driver circuit 222 has a function of writing data to the memory cell212 and a function of reading out data stored in the memory cell 212.Specifically, the driver circuit 222 has a function of supplying apotential (hereinafter also referred to as a writing potential)corresponding to data stored in the memory cell 212 to the wiring BLconnected to the memory cell 212 to which data is to be written.Furthermore, the driver circuit 222 has a function of reading apotential corresponding to data stored in the memory cell 212(hereinafter also referred to as a reading potential) and outputting thepotential to the control circuit 111 through the wiring CL

An address signal, a clock signal, a timing signal, and the like areinput from the control circuit 111 provided in the layer 100 to thedriver circuit 221 through the wiring CL. The driver circuit 221generates a selection signal with the use of these signals. Note thatthe timing at which a selection signal is output from the driver circuit221 is controlled by a timing signal input from the control circuit 111.

In addition, an address signal, a clock signal, a timing signal, data tobe written to the memory cell 212, and the like are supplied from thecontrol circuit 111 provided in the layer 100 to the driver circuit 222through the wiring CL. The driver circuit 222 generates writingpotentials with the use of these signals. Note that the timing at whichthe writing potential is output from the driver circuit 222 iscontrolled by the timing signal input from the control circuit 111.

Note that in FIG. 1(B), wirings CL connected to the driver circuit 221and the driver circuit 222 are illustrated as one wiring.

The memory cell 212, the driver circuit 221, and the driver circuit 222are formed with OS transistors. An oxide semiconductor has a bandgap of2.5 eV or larger, preferably 3.0 V or larger; thus, an OS transistor hasa low leakage current due to thermal excitation and also has anextremely low off-state current. Note that off-state current refers tocurrent that flows between a source and a drain when a transistor isoff.

An oxide semiconductor used in a channel formation region of atransistor is preferably an oxide semiconductor containing at least oneof indium (In) and zinc (Zn). Typical examples of such an oxidesemiconductor include an In-M-Zn oxide (an element M is Al, Ga, Y, orSn, for example). Reducing both impurities serving as electron donors,such as moisture or hydrogen, and oxygen vacancies can make an oxidesemiconductor i-type (intrinsic) or substantially i-type. Such an oxidesemiconductor can be referred to as a highly purified oxidesemiconductor. Note that the details of an OS transistor will bedescribed in Embodiment 3.

An OS transistor has an extremely low off-state current and thus issuitably used as a transistor included in the memory cell 212. Anoff-state current per micrometer of channel width of an OS transistorcan be, for example, lower than or equal to 100 zA/μm, lower than orequal to 10 zA/μm, lower than or equal to 1 zA/μm, or lower than orequal to 10 yA/μm. The use of an OS transistor in the memory cell 212enables data stored in the memory cell 212 can be retained for a longtime.

With the use of an OS transistor for the memory cell 212, the refreshrate of the memory cell 212 can be reduced. Alternatively, refreshoperation of the memory cell 212 can be unnecessary. In addition, thelow refresh rate of the memory cell 212 can reduce the power consumptionof the memory device 210. Alternatively, eliminating the need for therefresh operation of the memory cell 212 can reduce circuits needed forthe refresh operation.

Owing to an extremely low leakage current of an OS transistor, thememory cell 212 can retain multilevel data or analog data. Sinceoff-state current of the OS transistor is not easily increased even athigh temperatures, data stored in the memory cell 212 is less likely tobe lost even at high temperatures caused by heat generation by thesignal processing circuit 110 or the image sensor 310. The use of an OStransistor can increase the reliability of the memory device 210.

FIG. 2(A) is a circuit diagram illustrating a configuration example ofthe memory cell 212 including an OS transistor. The memory cell 212illustrated in FIG. 2(A) includes a transistor 213 and a capacitor 214.Note that “OS” in drawings indicates an OS transistor.

A gate of the transistor 213 is connected to a node a1, one of a sourceand a drain thereof is connected to one electrode of the capacitor 214,and the other of the source and the drain thereof is connected to a nodea2. The other electrode of the capacitor 214 is connected to a node a3to which a constant potential (e.g., a low power supply potential) issupplied. The node a1 is connected to the driver circuit 221 in FIG.1(B), and the node a2 is connected to the driver circuit 222 in FIG.1(B). Note that a node that is connected to the one of the source andthe drain of the transistor 213 and the one electrode of the capacitor214 is referred to as a node N1.

When data is written to the memory cell 212, a writing potential issupplied to the node a2. Then, a selection signal (high-level potential)is supplied to the node a1 to turn on the transistor 213. As a result,the writing potential is written to the node N1. After that, a low-levelpotential is supplied to the node a1 to turn off the transistor 213. Asa result, the node N1 is brought into a floating state and the writingpotential is retained.

When data stored in the memory cell 212 is read out, the potential ofthe node N1 is a reading potential. A selection signal (high-levelpotential) is supplied to the node a1 to turn on the transistor 213.Accordingly, the potential of the node a2 is determined in accordancewith the potential of the node N1. In this manner, data stored in thememory cell 212 is read out.

Since an OS transistor is used as the transistor 213, the potential ofthe node N1 is held for a long period. Accordingly, the data refreshrate can be reduced and thus power consumption can be reduced. Thememory device 210 including the memory cells 212 with a circuitillustrated in FIG. 2(A) is referred to as a DOSRAM in thisspecification and the like.

For the memory cell 212, a transistor having a back gate may be used.FIG. 2(B) illustrates a configuration example of the memory cell 212including a transistor 215 having a back gate. The memory cell 212illustrated in FIG. 2(B) includes the transistor 215 and the capacitor214.

The back gate of the transistor 215 is connected to a node a4. Byapplying a given potential to the node a4, a threshold voltage of thetransistor 215 can be increased or decreased. For example, when anegative potential (a potential which is lower than the potentials ofthe node a2 and the node N1) is applied to the back gate, the thresholdvoltage can be increased and the off-state current can be reduced.

As illustrated in FIG. 2(C), the back gate of the transistor 215 may beconnected to the node a1. When the back gate of the transistor 215 isconnected to the node a1, the amount of current flowing through thetransistor 215 can be increased when the transistor 215 is in an onstate. Note that the description except for the back gate in FIGS. 2(B)and 2(C) is similar to that in FIG. 2(A) and thus omitted.

Like the memory cell 212, the driver circuit 221 and the driver circuit222 illustrated in FIG. 1(B) are also formed with OS transistors. Thatis, the memory cell 212, the driver circuit 221, and the driver circuit222 do not include Si transistors and are formed with n-channel OStransistors. A circuit formed with transistors having the same polarityis hereinafter also referred to as a single-polarity circuit. That is,the layer 200 includes the memory device 210 including thesingle-polarity circuit using OS transistors.

Note that the control circuit 111 for controlling the memory device 210is provided in the layer 100, and can be formed with a CMOS circuit orthe like including Si transistors. Accordingly, the control circuit 111with high operation speed and high performance can be formed and thememory device 210 can be operated with the use of the control circuit111.

Although the configuration in which OS transistors are used in thecircuit provided in the layer 200 is described above, a transistor whosechannel formation region is formed in a film containing a semiconductormaterial other than an oxide semiconductor may be used. Examples of sucha transistor include a transistor using an amorphous silicon film, amicrocrystalline silicon film, a polycrystalline silicon film, a singlecrystal silicon film, an amorphous germanium film, a microcrystallinegermanium film, a polycrystalline germanium film, or a single crystalgermanium film for a semiconductor layer.

<Image Sensor 310>

The image sensor 310 is formed using a transistor formed over asemiconductor substrate SUB2. The semiconductor substrate SUB2 is notparticularly limited as long as a channel region of a transistor can beformed thereon. The description of the semiconductor substrate SUB2 issimilar to that of the semiconductor substrate SUB1 and thus is omitted.In this embodiment, a case where a single crystal silicon substrate isused as the semiconductor substrate SUB2 will be described as anexample.

The image sensor 310 is connected to the control circuit 112 byelectrical connections between the wiring CL that connects the layer 100to the layer 200, a conductor 201 provided on an outermost surface ofthe layer 200, and a conductor 301 provided on an outermost surface ofthe layer 300. Input/output of data and signals between the image sensor310 and the control circuit 112 is performed through the wiring CL, theconductor 201, and the conductor 301.

Here, the surface of the layer 200 is a surface where the memory device210 is formed, and the surface of the layer 300 is a surface where theimage sensor 310 is formed. In other words, the layer 300 is stackedover the layer 200 such that the surface where the image sensor 310 isformed is in contact with the surface where the memory device 210 isformed.

The image sensor 310 includes a pixel array 311, a driver circuit 321,and a driver circuit 322. The pixel array 311 includes a plurality ofpixels 312 arranged in a matrix.

The pixel 312 has a function of converting light intensity into anelectric signal. An electric signal obtained by the plurality of pixels312 is read out by the driver circuit 322 and output as image data fromthe image sensor 310 to the control circuit 112.

Here, since the layer 300 is stacked such that the surface where theimage sensor 310 is formed is in contact with the surface where thememory device 210 is formed, light 20 enters from a surface of the layer300 where the image sensor 310 is not formed (see FIG. 1(B)). For thisreason, the layer 300 is thin enough to transmit light.

The driver circuit 321 has a function of selecting the pixels 312.Specifically, the driver circuit 321 has a function of supplying aselection signal for selecting the pixels 312 on which data reading isto be performed, to a wiring connected to the pixels 312.

The driver circuit 322 has a function of reading out an electric signalfrom the pixel 312. The driver circuit 322 may have a function ofperforming noise removal, analog-digital conversion, or the like withrespect to the read-out electric signal. For example, in some cases, aCDS (Correlated Double Sampling) circuit may be included as a circuitfor noise removal, and a column-parallel (column type) analog-digitalconversion circuit may be included as a circuit for performinganalog-digital conversion.

The analog-digital conversion circuit is provided in the driver circuit322 or the control circuit 112 provided in the layer 100. Alternatively,the analog-digital conversion circuit may be provided in each of thedriver circuit 322 and the control circuit 112. With the use of theanalog-digital conversion circuit, image data generated by the imagesensor 310 can be processed as a digital signal.

A clock signal, a timing signal, and the like are input from the controlcircuit 112 provided in the layer 100 to the driver circuit 321 throughthe wiring CL, the conductor 201, and the conductor 301. The drivercircuit 321 generates a selection signal with the use of these signals.Note that the timing at which a selection signal is output from thedriver circuit 321 is controlled by the timing signal input from thecontrol circuit 112.

The driver circuit 322 has a function of outputting an electric signalread out from the pixel 312 as image data. The driver circuit 322outputs image data to the control circuit 112 through the conductor 301,the conductor 201, and the wiring CL.

Note that in FIG. 1(B), the wirings CL connected to the driver circuit321 and the driver circuit 322 are illustrated as one wiring, and theconductors 201 and the conductors 301 are each collectively illustrated.

<Pixel 312>

FIG. 2(D) is a circuit diagram illustrating a configuration example ofthe pixel 312. The pixel 312 illustrated in FIG. 2(D) includes aphotoelectric conversion element 313, a transistor 314, a transistor315, a transistor 316, a transistor 317, and a capacitor 318.

As the photoelectric conversion element 313, for example, a pn-junctionphotodiode using a p-type silicon semiconductor and an n-type siliconsemiconductor can be used. The photoelectric conversion element 313 maybe a pin-junction photodiode in which an i-type silicon semiconductorlayer is provided between a p-type silicon semiconductor and an n-typesilicon semiconductor. Alternatively, a pin diode element using anamorphous silicon film or a microcrystalline silicon film, adiode-connected transistor, or a variable resistor utilizing aphotoelectric effect may be formed using silicon, germanium, selenium,or the like.

A material capable of generating electric charge by absorbing radiationmay be used for the photoelectric conversion element 313. Examples ofthe material capable of generating electric charge by absorbingradiation include lead iodide, mercury iodide, gallium arsenide, CdTe,and CdZn.

One of a source and a drain of the transistor 314 is electricallyconnected to a cathode of the photoelectric conversion element 313, andthe other thereof is electrically connected to the node 331 (chargeaccumulation portion). An anode of the photoelectric conversion element313 is electrically connected to a wiring 334.

One of a source and a drain of the transistor 315 is electricallyconnected to the node 331, and the other of the source and the drain ofthe transistor 315 is electrically connected to a wiring 332. A gate ofthe transistor 316 is electrically connected to the node 331, one of asource and a drain of the transistor 316 is electrically connected to awiring 333, and the other of the source and the drain of the transistor316 is electrically connected to one of a source and a drain of thetransistor 317. The other of the source and the drain of the transistor317 is electrically connected to the wiring 332. One electrode of thecapacitor 318 is electrically connected to the node 331, and the otherelectrode of the capacitor 318 is electrically connected to the wiring334.

The transistor 314 has a function as a transfer transistor. A gate ofthe transistor 314 is supplied with a transfer signal TX. The transistor315 has a function as a reset transistor. A gate of the transistor 315is supplied with a reset signal RST. The transistor 316 has a functionas an amplifier transistor. The transistor 317 has a function as aselection transistor. A gate of the transistor 317 is supplied with aselection signal SEL.

A high power supply potential VDD is supplied to the wiring 332, and alow power supply potential VSS is supplied to the wiring 334. Here, inthis specification and the like, a high power supply potential is apower supply potential higher than a low power supply potential. The lowpower supply potential is a power supply potential lower than the highpower supply potential.

Next, operation of the pixel 312 illustrated in FIG. 2(D) will bedescribed.

First, the transistor 315 is turned on so that VDD is supplied to thenode 331 (reset operation). Then, the transistor 315 is turned off sothat VDD is retained at the node 331.

Next, the transistor 314 is turned on so that the potential of the node331 is changed in accordance with the amount of light received by thephotoelectric conversion element 313 (accumulation operation). Afterthat, the transistor 314 is turned off so that the potential of the node331 is retained.

Then, the transistor 317 is turned on so that a potential correspondingto the potential of the node 331 is output from the wiring 333(selection operation). By measuring the potential of the wiring 333, theamount of light received by the photoelectric conversion element 313 canbe determined.

The above operation is performed in all the pixels 312 included in thepixel array 311, and the driver circuit 322 reads out an electricsignal; thus, the image sensor 310 can generate image data.

Note that the above configuration of the pixel 312 is an example, anddoes not include part of the circuit, part of the transistors, part ofthe capacitor, or the like in some cases. Alternatively, a circuit, atransistor, a capacitor, or the like that is not included in the aboveconfiguration of the pixel 312 is included in some cases. Alternatively,some of the power supply potentials may be different.

<Imaging Device 10>

As described above, the imaging device 10 has a structure in which thelayer 200 is stacked over the layer 100 and the layer 300 is stackedover the layer 200, and includes the signal processing circuit 110, thememory device 210, and the image sensor 310. The imaging device 10performs analog-digital conversion, noise removal, or the other variousimage processing on the image data generated by the image sensor 310,and then can output data to an electronic device including the imagingdevice 10. Accordingly, the electronic device including the imagingdevice 10 can be reduced in size and weight.

The structure in which the imaging device 10 includes the memory device210 including OS transistors and data of a captured image is stored inthe memory device 210 enables high-speed imaging exceeding a speed oftransmitting and receiving data (a data transferring speed) between theinput/output circuit 114 and an electronic device including the imagingdevice 10.

Since OS transistors can be formed over the layer 100, the number ofsteps for bonding and steps for film thickness reduction can be smallerthan that in the method of manufacturing an imaging device described inNon-Patent Document 1. The imaging device 10 may include a plurality oflayers 200. FIG. 3 illustrates an example in which the imaging device 10includes two layers 200. An imaging device 15 illustrated in FIG. 3includes the layer 100, a layer 200 a, the layer 200, and the layer 300.Although the description of the layer 200 a, which is the same as thelayer 200 except that it does not include the conductor 201, is omitted,the storage capacity of the imaging device 10 can be easily increased byan increase in the number of layers 200.

Owing to an extremely low leakage current of the OS transistor includedin the memory device 210, data stored in the memory cell 212 can beretained for a long time. This can make the refresh rate of the memorycell 212 low or refresh operation unnecessary; accordingly, the powerconsumption of the memory device 210 can be reduced. Moreover, the OStransistor does not easily increase the off-state current even at hightemperatures; thus, data stored in the memory cell 212 is unlikely to belost even at high temperatures. That is, the reliability of the memorydevice 210 can be increased.

Note that this embodiment can be implemented in combination with theother embodiments described in this specification as appropriate.

Embodiment 2

In this embodiment, a cross-sectional structure example of the imagingdevice 10 described in the above embodiment will be described.

Note that the signal processing circuit 110 includes transistors formedon the semiconductor substrate SUB1 and the image sensor 310 includestransistors formed on the semiconductor substrate SUB2, that is,transistors of the layer 100 and transistors of the layer 300 are formedon the different semiconductor substrates. Since the memory device 210includes OS transistors and the layer 200 is formed over the layer 100,the layer 200 is formed above the semiconductor substrate SUB1.

FIG. 4 and FIG. 5 illustrate a cross-sectional structure example of thelayer 100 and the layer 200, and FIG. 6 illustrates a cross-sectionalstructure example of the layer 300.

<Layer 100 and Layer 200>

The cross-sectional structure example of the layers 100 and 200illustrated in FIG. 4 includes a transistor 400 a, a transistor 400 b, atransistor 500, and a capacitor 600. FIG. 5(A) is a cross-sectional viewof the transistor 500 in the channel length direction. FIG. 5(B) is across-sectional view of the transistor 500 in the channel widthdirection. FIG. 5(C) is a cross-sectional view of the transistor 400 ain the channel width direction.

The transistor 500 is a transistor including a metal oxide in itschannel formation region. Since the off-state current of the transistor500 is extremely low, the memory cell 212 including the transistor 500can retain stored data for a long time. In other words, powerconsumption of the memory device 210 can be reduced because the refreshoperation rate is low or the refresh operation is unnecessary.

As illustrated in FIG. 4 , the transistor 500 is provided above thetransistor 400 a and the transistor 400 b, and the capacitor 600 isprovided above the transistor 500.

The transistor 400 a is provided over a semiconductor substrate 411 andincludes a conductor 416, an insulator 415, a semiconductor region 413that is formed of part of the substrate 411, and a low-resistance region414 a and a low-resistance region 414 b functioning as a source regionand a drain region. Similarly, the transistor 400 b is provided on thesemiconductor substrate 411 and includes a conductor 416, an insulator415, a semiconductor region 417 that is formed of part of thesemiconductor substrate 411, and a low-resistance region 418 a and alow-resistance region 418 b functioning as a source region and a drainregion. Note that the semiconductor substrate 411 corresponds to thesemiconductor substrate SUB1 in Embodiment 1.

As illustrated in the cross-sectional view of the transistor 400 a inthe channel width direction in FIG. 5C, a top surface and a side surfacein the channel width direction of the semiconductor region 413 arecovered with the conductor 416 with the insulator 415 therebetween.Thus, the transistors 400 a and 400 b are Fin-type transistors. Theeffective channel width is increased in the Fin-type transistors used asthe transistors 400 a and 400 b, whereby the on-state characteristics ofthe transistors can be improved. In addition, since contribution ofelectric fields of a gate electrode can be increased, the off-statecharacteristics of the transistors 400 a and 400 b can be improved.

Note that in this embodiment, an n-type single crystal silicon substrateis used as the semiconductor substrate 411, for example. Thesemiconductor region 417 is part of a well of a p-type semiconductorprovided in part of the semiconductor substrate 411. That is, thetransistor 400 a functions as a p-channel transistor, and the transistor400 b functions as an n-channel transistor.

The semiconductor substrate 411 may be formed using a materialcontaining Ge (germanium), SiGe (silicon germanium), GaAs (galliumarsenide), GaAlAs (gallium aluminum arsenide), or the like. A structuremay be employed in which silicon whose effective mass is controlled byapplying stress to the crystal lattice and thereby changing the latticespacing is used. Alternatively, the transistor 400 may be an HEMT (HighElectron Mobility Transistor) with GaAs and GaAlAs, or the like.

The low-resistance regions 414 a and 414 b and the semiconductor region417 contain an element which imparts p-type conductivity, such as boron,in addition to a semiconductor material used for the semiconductorregion 413. The low-resistance regions 418 a and 418 b contain anelement which imparts n-type conductivity, such as arsenic orphosphorus, in addition to a semiconductor material used for thesemiconductor region 417.

The conductor 416 functioning as a gate electrode can be formed using asemiconductor material such as silicon containing an element thatimparts n-type conductivity, such as arsenic or phosphorus, or anelement that imparts p-type conductivity, such as boron, or using aconductive material such as a metal material, an alloy material, or ametal oxide material.

Note that since the work function of a conductor depends on a materialof the conductor, threshold voltage of the transistor can be adjusted bychanging the material of the conductor. Specifically, it is preferableto use a material such as titanium nitride or tantalum nitride for theconductor. Moreover, in order to ensure both conductivity andembeddability, it is preferable to use stacked layers of metal materialssuch as tungsten and aluminum for the conductor, and it is particularlypreferable to use tungsten in terms of heat resistance.

Note that the transistor 400 a and the transistor 400 b illustrated inFIG. 4 is just an example and the structure is not limited thereto; anappropriate transistor can be used in accordance with a circuitconfiguration or a driving method. For example, like the transistor 500,the transistor 400 a and the transistor 400 b may be formed using anoxide semiconductor.

An insulator 420, an insulator 422, an insulator 424, and an insulator426 are stacked in this order to cover the transistor 400 a and thetransistor 400 b.

The insulator 420, the insulator 422, the insulator 424, and theinsulator 426 can be formed using, for example, silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, aluminum oxide,aluminum oxynitride, aluminum nitride oxide, or aluminum nitride.

The insulator 422 may have a function as a planarization film forplanarizing a level difference caused by the transistor 400 a or thelike provided below the insulator 422. For example, a top surface of theinsulator 422 may be planarized by planarization treatment using achemical mechanical polishing (CMP) method or the like to improveplanarity.

The insulator 424 is preferably formed using a film having a barrierproperty that prevents diffusion of hydrogen or impurities from thesemiconductor substrate 411, the transistor 400 a, or the like into aregion where the transistor 500 is provided. For example, for the filmhaving a barrier property against hydrogen, silicon nitride formed by aCVD method can be used.

The diffusion of hydrogen to a semiconductor element including an oxidesemiconductor, such as the transistor 500, degrades the characteristicsof the semiconductor element in some cases. Therefore, a film thatinhibits hydrogen diffusion is preferably provided between thetransistor 500 and the transistors 400 a and 400 b. The film thatinhibits hydrogen diffusion is specifically a film from which a smallamount of hydrogen is released.

The amount of released hydrogen can be measured by thermal desorptionspectroscopy (TDS), for example. The amount of hydrogen released fromthe insulator 424 that is converted into hydrogen atoms per area of theinsulator 424 is less than or equal to 10×10¹⁵ atoms/cm², preferablyless than or equal to 5×10¹⁵ atoms/cm², in the TDS analysis in afilm-surface temperature range of 50° C. to 500° C., for example.

Note that the permittivity of the insulator 426 is preferably lower thanthat of the insulator 424. For example, the dielectric constant of theinsulator 426 is preferably lower than 4, further preferably lower than3. The dielectric constant of the insulator 426 is, for example,preferably 0.7 times or less, further preferably 0.6 times or less thedielectric constant of the insulator 424. When a material with a lowpermittivity is used for an interlayer film, the parasitic capacitancegenerated between wirings can be reduced.

A conductor 428, a conductor 430, and the like that are connected to thecapacitor 600 or the transistor 500 are embedded in the insulator 420,the insulator 422, the insulator 424, and the insulator 426. Note thatthe conductor 428 and the conductor 430 have a function as a plug or awiring.

Here, a plurality of conductors that has a function as plugs or wiringsare collectively denoted by the same reference numeral in some cases.Furthermore, in this specification and the like, a wiring and a plugconnected to the wiring may be a single component. That is, there arecases where part of a conductor functions as a wiring and another partof the conductor functions as a plug.

As a material of each of plugs and wirings (the conductor 428, theconductor 430, and the like), a single layer or stacked layers of aconductive material such as a metal material, an alloy material, a metalnitride material, or a metal oxide material can be used. It ispreferable to use a high-melting-point material that has both heatresistance and conductivity, such as tungsten or molybdenum, and it ispreferable to use tungsten. Alternatively, it is preferable to use alow-resistance conductive material such as aluminum or copper. The useof a low-resistance conductive material can reduce wiring resistance.

A wiring layer may be provided over the insulator 426 and the conductor430. For example, in FIG. 4 an insulator 450, an insulator 452, and aninsulator 454 are provided to be stacked in this order. Furthermore, aconductor 456 is formed in the insulator 450, the insulator 452, and theinsulator 454. The conductor 456 has a function as a plug or a wiringthat is connected to the transistor 400 a and the like. Note that theconductor 456 can be provided using a material similar to those for theconductor 428 and the conductor 430.

For example, like the insulator 424, the insulator 450 is preferablyformed using an insulator having a barrier property against hydrogen.Furthermore, the conductor 456 preferably contains a conductor having abarrier property against hydrogen. In this case, the conductor having abarrier property against hydrogen is formed in an opening of theinsulator 450 having a barrier property against hydrogen. With thisstructure, the transistor 400 a, the transistor 400 b, and thetransistor 500 can be separated by a barrier layer, so that thediffusion of hydrogen from the transistor 400 a and the transistor 400 binto the transistor 500 can be inhibited.

Note that for the conductor having a barrier property against hydrogen,tantalum nitride is preferably used, for example. The use of a stackincluding tantalum nitride and tungsten having high conductivity caninhibit the diffusion of hydrogen from the transistor 400 a and thetransistor 400 b while the conductivity of a wiring is kept. In thatcase, the tantalum nitride layer having a barrier property againsthydrogen is preferably in contact with the insulator 450 having abarrier property against hydrogen.

A wiring layer may be provided over the insulator 454 and the conductor456. For example, in FIG. 4 , an insulator 460, an insulator 462, and aninsulator 464 are provided to be stacked in this order. Furthermore, aconductor 466 is formed in the insulator 460, the insulator 462, and theinsulator 464. The conductor 466 has a function as a plug or a wiring.Note that the conductor 466 can be provided using a material similar tothose for the conductor 428 and the conductor 430.

For example, like the insulator 424, the insulator 460 is preferablyformed using an insulator having a barrier property against hydrogen.Furthermore, the conductor 466 preferably contains a conductor having abarrier property against hydrogen. In this case, the conductor having abarrier property against hydrogen is formed in an opening of theinsulator 460 having a barrier property against hydrogen. With thisstructure, the transistor 400 a, the transistor 400 b, and thetransistor 500 can be separated by a barrier layer, so that thediffusion of hydrogen from the transistor 400 a and the transistor 400 binto the transistor 500 can be inhibited.

Although a wiring layer including the conductor 456 and a wiring layerincluding the conductor 466 are described in the above, thecross-sectional structures of the layer 100 and the layer 200 are notlimited to this. A wiring layer which is similar to the wiring layerincluding the conductor 456 may be provided, or three or more wiringlayers which are similar to the wiring layer including the conductor 456may be provided.

Here, a gate of the transistor 400 a is electrically connected to one ofa source and a drain of the transistor 500 through the conductor 428,the conductor 430, the conductor 456, the conductor 466, and the like.This series of wirings corresponds to the wiring CL in Embodiment 1.

An insulator 510, an insulator 512, an insulator 514, and an insulator516 are provided to be stacked in this order over the insulator 464. Asubstance having a barrier property against oxygen or hydrogen ispreferably used for one of the insulator 510, the insulator 512, theinsulator 514, and the insulator 516.

For example, the insulator 510 and the insulator 514 are preferablyformed using a film having a barrier property that prevents diffusion ofhydrogen or impurities from the semiconductor substrate 411, the regionswhere the transistor 400 a and the transistor 400 b are provided, or thelike into the region where the transistor 500 is provided. Therefore, amaterial similar to that for the insulator 424 can be used.

For the film having a barrier property against hydrogen used as theinsulator 510 and the insulator 514, for example, a metal oxide such asaluminum oxide, hafnium oxide, or tantalum oxide is preferably used.

In particular, aluminum oxide has a high blocking effect that inhibitsthe passage of both oxygen and impurities such as hydrogen and moisturewhich are factors of a change in electrical characteristics of thetransistor. Accordingly, aluminum oxide can prevent the entry ofimpurities such as hydrogen and moisture into the transistor 500 in thefabrication process and after the fabrication of the transistor. Inaddition, release of oxygen from the oxide included in the transistor500 can be inhibited. Therefore, aluminum oxide is suitably used for aprotective film of the transistor 500.

The insulator 512 and the insulator 516 can be formed using a materialsimilar to that for the insulator 420, for example. When a material witha relatively low permittivity is used for the insulator, the parasiticcapacitance between wirings can be reduced. A silicon oxide film, asilicon oxynitride film, or the like can be used as the insulator 512and the insulator 516, for example.

A conductor 518, a conductor included in the transistor 500 (a conductor503 (see FIG. 5(A))), and the like are embedded in the insulator 510,the insulator 512, the insulator 514, and the insulator 516. Note thatthe conductor 518 functions as a plug or a wiring that is connected tothe capacitor 600 or the transistor 400 a. The conductor 518 can beprovided using a material similar to those for the conductor 428 and theconductor 430.

In particular, the conductor 518 in a region in contact with theinsulator 510 and the insulator 514 is preferably a conductor having abarrier property against oxygen, hydrogen, and water. With thisstructure, the transistors 400 a and 400 b and the transistor 500 can beseparated by the layer having a barrier property against oxygen,hydrogen, and water; thus, the diffusion of hydrogen and the like fromthe transistor 400 a and the transistor 400 b into the transistor 500can be inhibited.

The transistor 500 is provided above the insulator 516.

As illustrated in FIGS. 5(A) and 5(B), the transistor 500 includes theconductor 503 positioned to be embedded in the insulator 514 and theinsulator 516; an insulator 520 positioned over the insulator 516 andthe conductor 503; an insulator 522 positioned over the insulator 520;an insulator 524 positioned over the insulator 522; an oxide 530 apositioned over the insulator 524; an oxide 530 b positioned over theoxide 530 a; a conductor 542 a and a conductor 542 b positioned apartfrom each other over the oxide 530 b; an insulator 580 that ispositioned over the conductor 542 a and the conductor 542 b and includesan opening overlapping a region between the conductor 542 a and theconductor 542 b; a conductor 560 positioned in the opening; an insulator550 positioned between the conductor 560 and the oxide 530 b, theconductor 542 a, the conductor 542 b, and the insulator 580; and anoxide 530 c positioned between the insulator 550 and the oxide 530 b,the conductor 542 a, the conductor 542 b, and the insulator 580.

As illustrated in FIGS. 5(A) and 5(B), an insulator 544 is preferablypositioned between the insulator 580 and the oxide 530 a, the oxide 530b, the conductor 542 a, and the conductor 542 b. In addition, asillustrated in FIGS. 5(A) and 5(B), the conductor 560 preferablyincludes a conductor 560 a provided inside the insulator 550 and aconductor 560 b embedded inside the conductor 560 a. Moreover, asillustrated in FIGS. 5(A) and 5(B), an insulator 574 is preferablypositioned over the insulator 580, the conductor 560, and the insulator550.

Hereinafter, the oxide 530 a, the oxide 530 b, and the oxide 530 c maybe collectively referred to as an oxide 530. The conductor 542 a and theconductor 542 b may be collectively referred to as a conductor 542.

The transistor 500 has a structure in which three layers of the oxide530 a, the oxide 530 b, and the oxide 530 c are stacked in the regionwhere the channel is formed and its vicinity; however, the presentinvention is not limited thereto. For example, a single layer of theoxide 530 b, a two-layer structure of the oxide 530 b and the oxide 530a, a two-layer structure of the oxide 530 b and the oxide 530 c, or astacked-layer structure of four or more layers may be provided. Althoughthe conductor 560 is shown to have a stacked-layer structure of twolayers in the transistor 500, the present invention is not limitedthereto. For example, the conductor 560 may have a single-layerstructure or a stacked-layer structure of three or more layers. Notethat the transistor 500 illustrated in FIG. 4 and FIGS. 5(A) and 5(B) isan example, and the structure is not limited thereto; an appropriatetransistor can be used in accordance with a circuit configuration or adriving method.

Here, the conductor 560 functions as a gate electrode of the transistor,and the conductor 542 a and the conductor 542 b function as a sourceelectrode and a drain electrode. As described above, the conductor 560is formed to be embedded in the opening of the insulator 580 and theregion between the conductor 542 a and the conductor 542 b. Thepositions of the conductor 560, the conductor 542 a, and the conductor542 b are selected in a self-aligned manner with respect to the openingof the insulator 580. That is, in the transistor 500, the gate electrodecan be positioned between the source electrode and the drain electrodein a self-aligned manner. Therefore, the conductor 560 can be formedwithout an alignment margin, resulting in a reduction in the areaoccupied by the transistor 500. Accordingly, for example,miniaturization and high integration of the memory device 210 providedin the layer 200 can be achieved.

In addition, since the conductor 560 is formed in the region between theconductor 542 a and the conductor 542 b in a self-aligned manner, theconductor 560 does not have a region overlapping the conductor 542 a orthe conductor 542 b. Thus, parasitic capacitance formed between theconductor 560 and each of the conductor 542 a and the conductor 542 bcan be reduced. As a result, the switching speed of the transistor 500can be increased, and the transistor 500 can have high frequencycharacteristics.

The conductor 560 sometimes functions as a first gate (also referred toas top gate) electrode. The conductor 503 sometimes functions as asecond gate (also referred to as bottom gate) electrode. In that case,the threshold voltage of the transistor 500 can be controlled bychanging a potential applied to the conductor 503 independently of apotential applied to the conductor 560. In particular, the thresholdvoltage of the transistor 500 can be higher than 0 V and the off-statecurrent can be reduced by applying a negative potential to the conductor503. Thus, a drain current at the time when a potential applied to theconductor 560 is 0 V can be lower in the case where a negative potentialis applied to the conductor 503 than in the case where a negativepotential is not applied to the conductor 503.

The conductor 503 is positioned to be overlapped by the oxide 530 andthe conductor 560. Thus, when potentials are applied to the conductor560 and the conductor 503, an electric field generated from theconductor 560 and an electric field generated from the conductor 503 areconnected, so that the channel formation region formed in the oxide 530can be covered. In this specification and the like, a transistorstructure in which a channel formation region is electrically surroundedby electric fields of a first gate electrode and a second gate electrodeis referred to as a surrounded channel (S-channel) structure.

The conductor 503 has a structure similar to that of the conductor 518;a conductor 503 a is formed in contact with an inner wall of an openingin the insulator 514 and the insulator 516, and a conductor 503 b isformed on the inner side.

The insulator 520, the insulator 522, the insulator 524, and theinsulator 550 have a function of a gate insulator.

Here, as the insulator 524 in contact with the oxide 530, an insulatorthat contains oxygen more than oxygen in the stoichiometric compositionis preferably used. That is, an excess-oxygen region is preferablyformed in the insulator 524. When such an insulator containing excessoxygen is provided in contact with the oxide 530, oxygen vacancies inthe oxide 530 can be reduced and the reliability of the transistor 500can be improved.

As the insulator including an excess-oxygen region, specifically, anoxide that releases part of oxygen by heating is preferably used. Anoxide that releases oxygen by heating is an oxide film in which theamount of released oxygen converted into oxygen atoms is greater than orequal to 1.0×10¹⁸ atoms/cm³, preferably greater than or equal to1.0×10¹⁹ atoms/cm³, further preferably greater than or equal to 2.0×10¹⁹atoms/cm³ or greater than or equal to 3.0×10²⁰ atoms/cm³ in thermaldesorption spectroscopy analysis (TDS analysis). Note that thetemperature of the film surface in the TDS analysis is preferably higherthan or equal to 100° C. and lower than or equal to 700° C., or higherthan or equal to 100° C. and lower than or equal to 400° C.

In the case where the insulator 524 includes an excess-oxygen region, itis preferred that the insulator 522 have a function of inhibitingdiffusion of oxygen (e.g., an oxygen atom, an oxygen molecule, or thelike) (or that the insulator 522 do not easily transmit the aboveoxygen).

When the insulator 522 has a function of inhibiting diffusion of oxygenor impurities, oxygen contained in the oxide 530 is not diffused to theinsulator 520 side, which is preferable. Furthermore, the conductor 503can be prevented from reacting with oxygen contained in the insulator524 or the oxide 530.

For example, the insulator 522 is preferably formed using a single layeror stacked layers of an insulator containing what is called a high-kmaterial such as aluminum oxide, hafnium oxide, tantalum oxide,zirconium oxide, lead zirconate titanate (PZT), strontium titanate(SrTiO₃), or (Ba,Sr)TiO₃(BST). With miniaturization and high integrationof transistors, a problem such as leakage current may arise because of athinner gate insulator. When a high-k material is used for an insulatorfunctioning as the gate insulator, a gate potential during operation ofthe transistor can be reduced while the physical thickness of the gateinsulator is kept.

It is particularly preferable to use an insulator containing an oxide ofone or both of aluminum and hafnium, which is an insulating materialhaving a function of inhibiting diffusion of impurities, oxygen, and thelike (or an insulating material through which the oxygen is less likelyto pass). As the insulator containing an oxide of one or both ofaluminum and hafnium, aluminum oxide, hafnium oxide, an oxide containingaluminum and hafnium (hafnium aluminate), or the like is preferablyused. In the case where the insulator 522 is formed using such amaterial, the insulator 522 functions as a layer that inhibits releaseof oxygen from the oxide 530 and entry of impurities such as hydrogenfrom the periphery of the transistor 500 into the oxide 530.

Alternatively, aluminum oxide, bismuth oxide, germanium oxide, niobiumoxide, silicon oxide, titanium oxide, tungsten oxide, yttrium oxide, orzirconium oxide may be added to these insulators, for example.Alternatively, these insulators may be subjected to nitriding treatment.Silicon oxide, silicon oxynitride, or silicon nitride may be stackedover the insulator.

It is preferable that the insulator 520 be thermally stable. Forexample, when an insulator which is a high-k material is combined withsilicon oxide or silicon oxynitride, the insulator 520 having astacked-layer structure that has thermal stability and a high dielectricconstant can be obtained because silicon oxide and silicon oxynitridehave thermal stability.

Note that the insulator 520, the insulator 522, and the insulator 524may each have a stacked-layer structure of four or more layers. In thatcase, without limitation to a stacked-layer structure formed of the samematerial, a stacked-layer structure formed of different materials may beemployed.

In the transistor 500, a metal oxide functioning as an oxidesemiconductor is preferably used as the oxide 530 including a channelformation region. For example, as the oxide 530, a metal oxide such asan In—M—Zn oxide (the element M is one or more kinds selected fromaluminum, gallium, yttrium, copper, vanadium, beryllium, boron,titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum,cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the like)is preferably used. Furthermore, as the oxide 530, an In—Ga oxide or anIn—Zn oxide may be used.

The metal oxide functioning as the channel formation region in the oxide530 has a band gap of preferably 2 eV or higher, further preferably 2.5eV or higher. With the use of a metal oxide having such a wide band gap,the off-state current of the transistor can be reduced.

When the oxide 530 includes the oxide 530 a under the oxide 530 b, it ispossible to inhibit diffusion of impurities into the oxide 530 b fromthe components formed below the oxide 530 a. Moreover, including theoxide 530 c over the oxide 530 b makes it possible to inhibit diffusionof impurities into the oxide 530 b from the components formed above theoxide 530 c.

Note that the oxide 530 preferably has a stacked-layer structure ofoxides that differ in the atomic ratio of metal atoms. Specifically, theatomic ratio of the element M to the constituent elements in the metaloxide used for the oxide 530 a is preferably greater than the atomicratio of the element M to the constituent elements in the metal oxideused for the oxide 530 b. Moreover, the atomic ratio of the element M toIn in the metal oxide used for the oxide 530 a is preferably greaterthan the atomic ratio of the element M to In in the metal oxide used forthe oxide 530 b. Furthermore, the atomic ratio of In to the element Minthe metal oxide used for the oxide 530 b is preferably greater than theatomic ratio of In to the element M in the metal oxide used for theoxide 530 a. A metal oxide that can be used for the oxide 530 a or theoxide 530 b can be used for the oxide 530 c.

The energy of the conduction band minimum of each of the oxide 530 a andthe oxide 530 c is preferably higher than the energy of the conductionband minimum of the oxide 530 b. In other words, the electron affinityof each of the oxide 530 a and the oxide 530 c is preferably smallerthan the electron affinity of the oxide 530 b.

The energy level of the conduction band minimum gradually changes atjunction portions of the oxide 530 a, the oxide 530 b, and the oxide 530c. In other words, the energy level of the conduction band minimum atthe junction portions of the oxide 530 a, the oxide 530 b, and the oxide530 c continuously changes or is continuously connected. To obtain this,the density of defect states in a mixed layer formed at an interfacebetween the oxide 530 a and the oxide 530 b and an interface between theoxide 530 b and the oxide 530 c is preferably made low.

Specifically, when the oxide 530 a and the oxide 530 b or the oxide 530b and the oxide 530 c contain the same element (as a main component) inaddition to oxygen, a mixed layer with a low density of defect statescan be formed. For example, in the case where the oxide 530 b is anIn—Ga—Zn oxide, an In—Ga—Zn oxide, a Ga—Zn oxide, gallium oxide, or thelike is preferably used for the oxide 530 a and the oxide 530 c.

At this time, the oxide 530 b serves as a main carrier path. When theoxide 530 a and the oxide 530 c have the above structure, the density ofdefect states at the interface between the oxide 530 a and the oxide 530b and the interface between the oxide 530 b and the oxide 530 c can bemade low. Thus, the influence of interface scattering on carrierconduction is small, and the transistor 500 can have a high on-statecurrent.

The conductor 542 (the conductor 542 a and the conductor 542 b)functioning as the source electrode and the drain electrode is providedover the oxide 530 b. For the conductor 542, it is preferable to use ametal element selected from aluminum, chromium, copper, silver, gold,platinum, tantalum, nickel, titanium, molybdenum, tungsten, hafnium,vanadium, niobium, manganese, magnesium, zirconium, beryllium, indium,ruthenium, iridium, strontium, and lanthanum; an alloy containing any ofthe above metal elements; an alloy containing a combination of the abovemetal elements; or the like. For example, it is preferable to usetantalum nitride, titanium nitride, tungsten, a nitride containingtitanium and aluminum, a nitride containing tantalum and aluminum,ruthenium oxide, ruthenium nitride, an oxide containing strontium andruthenium, an oxide containing lanthanum and nickel, or the like.Tantalum nitride, titanium nitride, a nitride containing titanium andaluminum, a nitride containing tantalum and aluminum, ruthenium oxide,ruthenium nitride, an oxide containing strontium and ruthenium, and anoxide containing lanthanum and nickel are preferable because they areoxidation-resistant conductive materials or materials that retain theirconductivity even after absorbing oxygen.

As illustrated in FIG. 5(A), a region 543 (a region 543 a and a region543 b) is sometimes formed as a low-resistance region at and near theinterface between the oxide 530 and the conductor 542. In that case, theregion 543 a functions as one of a source region and a drain region, andthe region 543 b functions as the other of the source region and thedrain region. The channel formation region is formed in a region betweenthe region 543 a and the region 543 b.

When the conductor 542 is provided in contact with the oxide 530, theoxygen concentration in the region 543 sometimes decreases. In addition,a metal compound layer that contains the metal contained in theconductor 542 and the component of the oxide 530 is sometimes formed inthe region 543. In such a case, the carrier density of the region 543increases, and the region 543 becomes a low-resistance region.

The insulator 544 is provided to cover the conductor 542 and inhibitsoxidation of the conductor 542. At this time, the insulator 544 may beprovided to cover a side surface of the oxide 530 and to be in contactwith the insulator 524.

A metal oxide containing one or more kinds selected from hafnium,aluminum, gallium, yttrium, zirconium, tungsten, titanium, tantalum,nickel, germanium, magnesium, and the like can be used as the insulator544.

For the insulator 544, it is particularly preferable to use an insulatorcontaining an oxide of one or both of aluminum and hafnium, for example,aluminum oxide, hafnium oxide, or an oxide containing aluminum andhafnium (hafnium aluminate). In particular, hafnium aluminate has higherheat resistance than a hafnium oxide film. Therefore, hafnium aluminateis preferable because it is less likely to be crystallized by heattreatment in a later step. Note that the insulator 544 is not anessential component when the conductor 542 is an oxidation-resistantmaterial or does not significantly lose its conductivity even afterabsorbing oxygen. Design is appropriately set in consideration ofrequired transistor characteristics.

The insulator 550 functions as a gate insulator. The insulator 550 ispreferably positioned in contact with the inner side (the top surfaceand the side surface) of the oxide 530 c. The insulator 550 ispreferably formed using an insulator from which oxygen is released byheating. For example, the insulator 550 is an oxide film in which theamount of released oxygen converted into oxygen atoms is greater than orequal to 1.0×10¹⁸ atoms/cm³, preferably greater than or equal to1.0×10¹⁹ atoms/cm³, further preferably greater than or equal to 2.0×10¹⁹atoms/cm³ or greater than or equal to 3.0×10²⁰ atoms/cm³ in thermaldesorption spectroscopy analysis (TDS analysis). Note that thetemperature of the film surface in the TDS analysis is preferably higherthan or equal to 100° C. and lower than or equal to 700° C.

Specifically, silicon oxide containing excess oxygen, siliconoxynitride, silicon nitride oxide, silicon nitride, silicon oxide towhich fluorine is added, silicon oxide to which carbon is added, siliconoxide to which carbon and nitrogen are added, porous silicon oxide, orthe like can be used. In particular, silicon oxide and siliconoxynitride, which have thermal stability, are preferable.

When an insulator from which oxygen is released by heating is providedas the insulator 550 in contact with the top surface of the oxide 530 c,oxygen can be efficiently supplied from the insulator 550 to the channelformation region of the oxide 530 b through the oxide 530 c.Furthermore, as in the insulator 524, the concentration of impuritiessuch as water or hydrogen in the insulator 550 is preferably reduced.The thickness of the insulator 550 is preferably greater than or equalto 1 nm and less than or equal to 20 nm.

In order to efficiently supply excess oxygen in the insulator 550 to theoxide 530, a metal oxide may be provided between the insulator 550 andthe conductor 560. The metal oxide preferably inhibits diffusion ofoxygen from the insulator 550 to the conductor 560. Providing the metaloxide that inhibits diffusion of oxygen inhibits diffusion of excessoxygen from the insulator 550 to the conductor 560. That is, a reductionin the amount of excess oxygen supplied to the oxide 530 can beinhibited. Moreover, oxidization of the conductor 560 due to excessoxygen can be inhibited. For the metal oxide, a material that can beused for the insulator 544 is used.

Although the conductor 560 functioning as the first gate electrode has atwo-layer structure in FIGS. 5(A) and 5(B), a single-layer structure ora stacked-layer structure of three or more layers may be employed.

For the conductor 560 a, it is preferable to use a conductive materialhaving a function of inhibiting diffusion of impurities such as ahydrogen atom, a hydrogen molecule, a water molecule, a nitrogen atom, anitrogen molecule, a nitrogen oxide molecule (N₂O, NO, NO₂, and thelike), and a copper atom. Alternatively, it is preferable to use aconductive material having a function of inhibiting diffusion of oxygen(e.g., an oxygen atom, an oxygen molecule, and the like). When theconductor 560 a has a function of inhibiting oxygen diffusion, it ispossible to prevent a reduction in conductivity of the conductor 560 bdue to oxidation caused by oxygen contained in the insulator 550. As aconductive material having a function of inhibiting oxygen diffusion,for example, tantalum, tantalum nitride, ruthenium, ruthenium oxide, orthe like is preferably used.

The conductor 560 b is preferably formed using a conductive materialcontaining tungsten, copper, or aluminum as its main component. Theconductor 560 b also functions as a wiring and thus is preferably formedusing a conductor having high conductivity. For example, a conductivematerial containing tungsten, copper, or aluminum as its main componentcan be used. The conductor 560 b may have a stacked-layer structure, forexample, a stacked-layer structure of any of the above conductivematerials and titanium or titanium nitride.

The insulator 580 is provided over the conductor 542 with the insulator544 therebetween. The insulator 580 preferably includes an excess-oxygenregion. For example, the insulator 580 preferably contains siliconoxide, silicon oxynitride, silicon nitride oxide, silicon nitride,silicon oxide to which fluorine is added, silicon oxide to which carbonis added, silicon oxide to which carbon and nitrogen are added, poroussilicon oxide, a resin, or the like. In particular, silicon oxide andsilicon oxynitride, which have thermal stability, are preferable. Inparticular, silicon oxide and porous silicon oxide, in which anexcess-oxygen region can be easily formed in a later step, arepreferable.

The insulator 580 preferably includes an excess-oxygen region. When theinsulator 580 from which oxygen is released by heating is provided incontact with the oxide 530 c, oxygen in the insulator 580 can beefficiently supplied to the oxide 530 through the oxide 530 c. Note thatthe concentration of impurities such as water or hydrogen in theinsulator 580 is preferably lowered.

The opening of the insulator 580 is formed to overlap a region betweenthe conductor 542 a and the conductor 542 b. Accordingly, the conductor560 is formed to be embedded in the opening of the insulator 580 and theregion between the conductor 542 a and the conductor 542 b.

The gate length needs to be short for miniaturization of thesemiconductor device, but it is necessary to prevent a reduction inconductivity of the conductor 560. When the conductor 560 is made thickto achieve this, the conductor 560 might have a shape with a high aspectratio. In this embodiment, the conductor 560 is provided to be embeddedin the opening of the insulator 580; hence, even when the conductor 560has a shape with a high aspect ratio, the conductor 560 can be formedwithout collapsing during the process.

The insulator 574 is preferably provided in contact with the top surfaceof the insulator 580, the top surface of the conductor 560, and the topsurface of the insulator 550. When the insulator 574 is deposited by asputtering method, excess-oxygen regions can be provided in theinsulator 550 and the insulator 580. Accordingly, oxygen can be suppliedfrom the excess-oxygen regions to the oxide 530.

For example, a metal oxide containing one or more kinds selected fromhafnium, aluminum, gallium, yttrium, zirconium, tungsten, titanium,tantalum, nickel, germanium, magnesium, and the like can be used as theinsulator 574.

In particular, aluminum oxide has a high barrier property, and even athin aluminum oxide film having a thickness of greater than or equal to0.5 nm and less than or equal to 3.0 nm can inhibit diffusion ofhydrogen and nitrogen. Accordingly, aluminum oxide deposited by asputtering method serves as an oxygen supply source and can also have afunction of a barrier film against impurities such as hydrogen.

An insulator 581 functioning as an interlayer film is preferablyprovided over the insulator 574. As in the insulator 524 or the like,the concentration of impurities such as water or hydrogen in theinsulator 581 is preferably lowered.

A conductor 540 a and a conductor 540 b are positioned in openingsformed in the insulator 581, the insulator 574, the insulator 580, andthe insulator 544. The conductor 540 a and the conductor 540 b areprovided to face each other with the conductor 560 therebetween. Thestructures of the conductor 540 a and the conductor 540 b are similar toa structure of a conductor 546 and a conductor 548 that will bedescribed later.

An insulator 582 is provided over the insulator 581. A substance havinga barrier property against oxygen or hydrogen is preferably used for theinsulator 582. Therefore, a material similar to that for the insulator514 can be used for the insulator 582. For the insulator 582, a metaloxide such as aluminum oxide, hafnium oxide, or tantalum oxide ispreferably used, for example.

In particular, aluminum oxide has a high blocking effect that inhibitsthe passage of both oxygen and impurities such as hydrogen and moisturewhich are factors of a change in electrical characteristics of thetransistor. Accordingly, aluminum oxide can prevent the entry ofimpurities such as hydrogen and moisture into the transistor 500 in thefabrication process and after the fabrication of the transistor. Inaddition, release of oxygen from the oxide included in the transistor500 can be inhibited. Therefore, aluminum oxide is suitably used for aprotective film of the transistor 500.

An insulator 586 is provided over the insulator 582. For the insulator586, a material similar to that for the insulator 420 can be used. Whena material with a relatively low permittivity is used for the insulator,the parasitic capacitance between wirings can be reduced. For example, asilicon oxide film, a silicon oxynitride film, or the like can be usedfor the insulator 586.

The conductor 546, the conductor 548, and the like are embedded in theinsulator 520, the insulator 522, the insulator 524, the insulator 544,the insulator 580, the insulator 574, the insulator 581, the insulator582, and the insulator 586.

The conductor 546 and the conductor 548 have functions as plugs orwirings that are connected to the capacitor 600, the transistor 500, orthe transistors 400 a and 400 b. The conductor 546 and the conductor 548can be provided using a material similar to those for the conductor 428and the conductor 430.

In addition, the capacitor 600 is provided above the transistor 500. Thecapacitor 600 includes a conductor 610, a conductor 620, and aninsulator 630.

A conductor 612 may be provided over the conductor 546 and the conductor548. The conductor 612 has a function of a plug or a wiring that isconnected to the transistor 500. The conductor 610 has a function of anelectrode of the capacitor 600. The conductor 612 and the conductor 610can be formed at the same time.

The conductor 612 and the conductor 610 can be formed using a metal filmcontaining an element selected from molybdenum, titanium, tantalum,tungsten, aluminum, copper, chromium, neodymium, and scandium; a metalnitride film containing any of the above elements as its component (atantalum nitride film, a titanium nitride film, a molybdenum nitridefilm, or a tungsten nitride film); or the like. Alternatively, it ispossible to use a conductive material such as indium tin oxide, indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium zinc oxide, or indium tin oxide towhich silicon oxide is added.

Although the conductor 612 and the conductor 610 each of which has asingle-layer structure are illustrated in FIG. 4 , the structure is notlimited thereto; a stacked-layer structure of two or more layers may beemployed. For example, between a conductor having a barrier property anda conductor having high conductivity, a conductor that is highlyadhesive to the conductor having a barrier property and the conductorhaving high conductivity may be formed.

The conductor 620 is provided to overlap the conductor 610 with theinsulator 630 therebetween. The conductor 620 can be formed using aconductive material such as a metal material, an alloy material, or ametal oxide material. It is preferable to use a high-melting-pointmaterial that has both heat resistance and conductivity, such astungsten or molybdenum, and it is particularly preferable to usetungsten. In the case where the conductor 620 is formed concurrentlywith another component such as a conductor, Cu (copper), Al (aluminum),or the like, which is a low-resistance metal material, can be used.

An insulator 650 is provided over the conductor 620 and the insulator630. The insulator 650 can be provided using a material similar to thatfor the insulator 420. The insulator 650 may function as a planarizationfilm that covers an uneven shape thereunder.

A conductor 646 and a conductor 648 are embedded in the insulator 650.The conductor 646 and the conductor 648 have a function as a plugconnected to the transistor 500, the transistor 400 a, the transistor400 b, and the like. The conductor 646 and the conductor 648 can beprovided using a material similar to those of the conductor 428 and theconductor 430.

A conductor 660 is provided over the conductor 646 and the conductor648. The conductor 660 can be provided using a material similar to thoseof the conductor 612 and the conductor 610. The conductor 660corresponds to the conductor 201 in Embodiment 1. In other words,electrical connection with the layer 300 can be obtained through theconductor 660.

Note that a layer including an element similar to the transistor 500 andthe capacitor 600 may be provided over the transistor 500 and thecapacitor 600. When a plurality of layers including the transistor 500and the capacitor 600 are provided, the storage capacity of the memorydevice 210 can be increased.

With the use of the above structure, a change in electricalcharacteristics can be inhibited and reliability can be improved in thelayer 200 including a transistor including an oxide semiconductor.Alternatively, a transistor including an oxide semiconductor and havinga high on-state current can be provided. Alternatively, a transistorincluding an oxide semiconductor and having a low off-state current canbe provided. Alternatively, the memory device 210 with low powerconsumption can be provided. Alternatively, the memory device 210 usinga transistor including an oxide semiconductor can be miniaturized orhighly integrated.

<Layer 300>

FIG. 6 illustrates a cross-sectional structure example of the layer 300.The cross-sectional structure example of the layer 300 in FIG. 6includes a transistor 700 a and a pn-junction photodiode 700 c. Notethat the photodiode 700 c functions as the photoelectric conversionelement 313 in Embodiment 1.

The transistor 700 a is provided over a semiconductor substrate 711 andincludes a conductor 716, an insulator 715, a semiconductor region 713that is formed of part of the semiconductor substrate 711, and alow-resistance region 714 a and a low-resistance region 714 bfunctioning as a source region and a drain region. Note that thesemiconductor substrate 711 corresponds to the semiconductor substrateSUB2 in Embodiment 1.

Note that in this embodiment, an n-type single crystal silicon substrateis used as the semiconductor substrate 711, for example. Part of thesemiconductor substrate 711 functions as an n-type semiconductor of thephotodiode 700 c and a p-type semiconductor region 718 provided in thesemiconductor substrate 711 functions as a p-type semiconductor of thephotodiode 700 c. The transistor 700 a functions as a p-channeltransistor. An n-channel transistor can be formed in a manner similar tothat of the above-described transistor 400 b though not beingillustrated in FIG. 6 .

The low-resistance region 714 a, the low-resistance region 714 b, andthe p-type semiconductor region 718 contain an element that impartsp-type conductivity, such as boron, in addition to a semiconductormaterial used for the semiconductor region 713.

The conductor 716 functioning as a gate electrode can be formed using asemiconductor material such as silicon containing the element whichimparts n-type conductivity, such as arsenic or phosphorus, or theelement which imparts p-type conductivity, such as boron, or using aconductive material such as a metal material, an alloy material, or ametal oxide material.

Note that the work function depends on a material of the conductor;thus, the threshold voltage of the transistor can be adjusted bychanging the material of the conductor. Specifically, it is preferableto use a material such as titanium nitride or tantalum nitride for theconductor. Moreover, in order to ensure both conductivity andembeddability, it is preferable to use a stacked layer of metalmaterials such as tungsten and aluminum for the conductor, and it isparticularly preferable to use tungsten in terms of heat resistance.

An insulator 720, an insulator 722, an insulator 724, and an insulator726 are sequentially stacked to cover the transistor 700 a and thephotodiode 700 c.

The insulator 720, the insulator 722, the insulator 724, and theinsulator 726 can be formed using, for example, silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, aluminum oxide,aluminum oxynitride, aluminum nitride oxide, or aluminum nitride.

The insulator 722 may have a function of a planarization film foreliminating a level difference caused by the transistor 700 a or thelike underlying the insulator 722. For example, a top surface of theinsulator 722 may be planarized by planarization treatment using achemical mechanical polishing (CMP) method or the like to improveplanarity. By the CMP treatment, unevenness of a sample surface can bereduced, and coverage with an insulating layer or a conductive layer tobe formed later can be increased. For the insulator 722, alow-dielectric constant material (a low-k material), a siloxane-basedresin, PSG (phosphosilicate glass), BPSG (borophosphosilicate glass), orthe like may be used.

A conductor 728, a conductor 730, and the like are embedded in theinsulator 720, the insulator 722, the insulator 724, and the insulator726. Note that the conductor 728 and the conductor 730 have a functionas plugs or wirings.

As a material of each of the plugs and wirings (the conductor 728, theconductor 730, and the like), a single layer or a stacked layer of aconductive material such as a metal material, an alloy material, a metalnitride material, or a metal oxide material can be used. It ispreferable to use a high-melting-point material that has both heatresistance and conductivity, such as tungsten or molybdenum, and it ispreferable to use tungsten. Alternatively, a low-resistance conductivematerial such as aluminum or copper is preferably used. The use of alow-resistance conductive material can reduce wiring resistance.

A wiring layer may be provided over the insulator 726 and the conductor730. For example, in FIG. 6 , an insulator 750, an insulator 752, and aninsulator 754 are stacked sequentially. Furthermore, a conductor 756 isformed in the insulator 750, the insulator 752, and the insulator 754.The conductor 756 has a function as a plug or a wiring that is connectedto the transistor 700 a or the like. Note that the conductor 756 can beprovided using a material similar to those for the conductor 728 and theconductor 730. The insulator 750, the insulator 752, and the insulator754 can be provided using a material similar to that for the insulator720 and the like.

A wiring layer may be provided over the insulator 754 and the conductor756. For example, in FIG. 6 , an insulator 760, an insulator 762, and aninsulator 764 are provided to be stacked in this order. Furthermore, aconductor 766 is formed in the insulator 760, the insulator 762, and theinsulator 764. The conductor 766 has a function as a plug or a wiring.Note that the conductor 766 can be provided using a material similar tothose for the conductor 728 and the conductor 730. The insulator 760,the insulator 762, and the insulator 764 can be formed using a materialsimilar to that for the insulator 720 or the like.

Although a wiring layer including the conductor 756 and a wiring layerincluding the conductor 766 are described in the above, thecross-sectional structure of the layer 300 is not limited to this. Awiring layer which is similar to the wiring layer including theconductor 756 may be provided, or three or more wiring layers which aresimilar to the wiring layer including the conductor 756 may be provided.

A conductor 770 is provided over the conductor 766. The conductor 770can be provided using a material similar to those for the conductor 728and the conductor 730. The conductor 770 corresponds to the conductor301 in Embodiment 1. In other words, electrical connection with thelayer 100 and the layer 200 can be obtained through the conductor 770.

<Imaging Device 10>

The imaging device 10 is formed by bonding the above-described layers100 and 200 to the layer 300. FIG. 7 illustrates a cross-sectionalstructure example of the imaging device 10.

The imaging device 10 is formed by bonding surfaces of the layer 100 andthe layer 200, over the semiconductor substrate 411, where thetransistor 400 a, the transistor 400 b, the transistor 500, thecapacitor 600, and the like are provided to a surface of the layer 300where the transistor 700 a, the photodiode 700 c, and the like areprovided. Note that the imaging device 10 illustrated in FIG. 7 isdifferent from the imaging device 10 in FIG. 4 and FIG. 6 in that somereference numerals, the layer including the conductor 466, and the layerincluding the conductor 756 are omitted and the scale of part of thedrawing is changed.

In FIG. 7 , the layers 100 and 200 are bonded to the layer 300, wherebythe conductor 660 and the conductor 770 are electrically connected.After the bonding of the layers 100 and 200 to the layer 300, thesemiconductor substrate 711 included in the layer 300 is reduced inthickness, whereby the imaging device 10 is formed.

The photodiode 700 c captures light transmitted through thesemiconductor substrate 711 and converts the light to an electricsignal. The electric signal converted by the photodiode 700 c isconverted into a digital signal in an analog-digital conversion circuitincluded in the signal processing circuit 110 or the image sensor 310.The electric signal converted by the photodiode 700 c is transmitted tothe signal processing circuit 110 through the conductor 770 and theconductor 660.

Structure Example 1 of Transistor

Although FIG. 4 and FIG. 5 illustrate a structure example in which theconductor 542 that functions as a source electrode or a drain electrodeis in contact with the oxide 530, the structure of the OS transistor isnot limited to this. For example, a structure in which the conductor 542is not provided and the resistance of the oxide 530 is selectivelyreduced so as to form a source region or a drain region in the oxide 530b can be employed. A structure example of such a transistor isillustrated in FIG. 8 .

FIG. 8(A) is a cross-sectional view of a transistor 500A in the channellength direction, and FIG. 8(B) is a cross-sectional view of thetransistor 500A in the channel width direction. Note that the transistor500A illustrated in FIG. 8 is a modification example of the transistor500 illustrated in FIG. 5 . Therefore, what is different from thetransistor 500 is mainly described to avoid repeated description.

In the transistor 500A, a metal oxide functioning as an oxidesemiconductor can be used as the oxide 530 including a channel formationregion, as in the transistor 500.

When an element that forms an oxygen vacancy or an element that isbonded to an oxygen vacancy is added to the oxide 530, the carrierdensity is increased and the resistance is lowered in some cases.Typical examples of an element that lowers the resistance of the oxide530 include boron and phosphorus. Moreover, hydrogen, carbon, nitrogen,fluorine, sulfur, chlorine, titanium, a rare gas, or the like may beused. Typical examples of the rare gas are helium, neon, argon, krypton,and xenon.

Note that the concentration of the above element is measured bysecondary ion mass spectrometry (SIMS) or the like.

Boron and phosphorus are particularly preferable because an apparatus ina manufacturing line for amorphous silicon or low-temperaturepolysilicon can be used. Using the existing facility can reduce capitalinvestment.

The region 543 (the region 543 a and the region 543 b) illustrated inFIG. 8 is a region where the above element is added to the oxide 530 b.The region 543 can be formed with the use of a dummy gate, for example.

For example, a dummy gate is provided over the oxide 530 b, and anelement that lowers the resistance of the oxide 530 b is added using thedummy gate as a mask. That is, the element is added to regions of theoxide 530 that are not overlapped by the dummy gate, whereby the region543 is formed. Note that as a method for adding the element, an ionimplantation method by which an ionized source gas is subjected to massseparation and then added, an ion doping method by which an ionizedsource gas is added without mass separation, a plasma immersion ionimplantation method, or the like can be used.

Next, an insulating film to be the insulator 544 and an insulating filmto be an insulator 545 may be formed over the oxide 530 b and the dummygate. The insulating film to be the insulator 544 and the insulatingfilm to be the insulator 545 are stacked, whereby a region where theregion 543 is overlapped by the oxide 530 c and the insulator 550 can beprovided.

Specifically, after an insulating film to be the insulator 580 isprovided over the insulating film to be the insulator 545, theinsulating film to be the insulator 580 is subjected to CMP (ChemicalMechanical Polishing) treatment, whereby part of the insulating film tobe the insulator 580 is removed and the dummy gate is exposed. Then,when the dummy gate is removed, part of the insulator 544 in contactwith the dummy gate is preferably also removed. Thus, the insulator 545and the insulator 544 are exposed at the side surface of the openingprovided in the insulator 580, and the region 543 provided in the oxide530 b is partly exposed at the bottom surface of the opening. Next, anoxide film to be the oxide 530 c, an insulating film to be the insulator550, and a conductive film to be the conductor 560 are formed in thisorder in the opening, and then, the oxide film to be the oxide 530 c,the insulating film to be the insulator 550, and the conductive film tobe the conductor 560 are partly removed by CMP treatment or the likeuntil the insulator 580 is exposed; thus, the transistor illustrated inFIG. 8 can be formed.

Note that the insulator 544 and the insulator 545 are not essentialcomponents. Design is appropriately set in consideration of requiredtransistor characteristics.

Since the existing device can be used as the transistor illustrated inFIG. 8 and the conductor 542 is not provided, a cost reduction can beachieved.

Structure Example 2 of Transistor

Although FIG. 4 and FIG. 5 illustrate a structure example in which theconductor 560 that functions as a gate is formed in an opening of theinsulator 580, the structure of the OS transistor is not limited tothis. For example, a structure in which the insulator is provided abovethe conductor can be employed. A structure example of such a transistoris illustrated in FIG. 9 and FIG. 10 .

FIG. 9(A) is a top view of a transistor and FIG. 9(B) is a perspectiveview of the transistor.

FIG. 10(A) is a cross-sectional view taken along X1-X2 in FIG. 9(A), andFIG. 10(B) is a cross-sectional view taken along Y1-Y2 in FIG. 9(A).

The transistor illustrated in FIG. 9 and FIG. 10 includes a conductorBGE having a function as a back gate, an insulator BGI having a functionas a gate insulating film, an oxide semiconductor S, an insulator FGIhaving a function as a gate insulating film, a conductor FGE having afunction as a front gate, and a conductor WE having a function as awiring. A conductor PE has a function as a plug for connecting theconductor WE to the oxide S, the conductor BGE, or the conductor FGE.Note that an example in which the oxide semiconductor S includes threelayers of oxides S1, S2, and S3 is shown here.

<Electrical Characteristics of Transistors>

Next, electrical characteristics of an OS transistor will be describedbelow. A transistor including a first gate and a second gate isdescribed below as an example. In the transistor including the firstgate and the second gate, the threshold voltage can be controlled byapplying different potentials to the first gate and the second gate. Forexample, by applying a negative potential to the second gate, thethreshold voltage of the transistor can be higher than 0 V and theoff-state current can be reduced. That is, when a negative potential isapplied to the second gate electrode, a drain current when the potentialapplied to the first electrode is 0 V can be reduced.

When impurity such as hydrogen is added to an oxide semiconductor, thecarrier density is increased in some cases. For example, hydrogen addedto an oxide semiconductor reacts with oxygen bonded to a metal atom tobe water, so that an oxygen vacancy is formed in some cases. Entry ofhydrogen into the oxygen vacancy increases carrier density. Furthermore,in some cases, bonding of part of hydrogen to oxygen bonded to a metalatom causes generation of an electron serving as a carrier. That is, theoxide semiconductor to which an impurity such as hydrogen is addedbecomes n-type and has a reduced resistance.

Therefore, the resistance of the oxide semiconductor can be selectivelyreduced. That is, a region which has a low carrier density and functionsas a semiconductor functioning as a channel formation region and alow-resistance region which has a high carrier density and functions asa source region or a drain region can be provided in the oxidesemiconductor.

Here, evaluated is the influence of the structure of a low-resistanceregion and a high-resistance region provided in the oxide semiconductoron electrical characteristics of the transistor in the case wheredifferent potentials are applied to the first gate and the second gate.

[Structure of Transistor]

FIGS. 11(A) and 11(C) are each a cross-sectional view of a transistorused for the electrical characteristics evaluation. For clarity of thedrawings, some components are not illustrated in FIGS. 11(A) and 11(C).

The transistor illustrated in FIGS. 11(A) and 11(C) includes a conductorTGE that functions as a first gate, an insulator TGI that functions as afirst gate insulating film, an insulator SW that functions as a sidewallprovided on a side surface of the first gate, an oxide semiconductor S,a conductor BGE that functions as a second gate, and an insulator BGIthat functions as a second gate insulator. The insulator BGI has athree-layer structure formed of a first layer in contact with theconductor BGE, a second layer over the first layer, and a third layerover the second layer. Note that the third layer is in contact with theoxide semiconductor S.

Here, the oxide semiconductor S included in the transistor illustratedin FIG. 11(A) has an n⁺ region and an i region overlapping with theconductor TGE. On the other hand, the oxide semiconductor S included inthe transistor illustrated in FIG. 11(C) has the n⁺ region, the i regionoverlapping with the conductor TGE, and an n⁻ region between the n⁺region and the i region.

Note that the n⁺ region functions as a source region or a drain regionand has a high carrier density and reduced resistance. The i regionfunctions as a channel formation region and is a high-resistance regionwhose carrier density is lower than the n⁺ region. Then region has alower carrier density than the n⁺ region and a higher carrier densitythan the i region.

Although not illustrated, the n⁺ region of the oxide semiconductor S isin contact with an S/D electrode functioning as a source or a drain.

[Results of Electrical Characteristics Evaluation]

The Id-Vg characteristics of the transistor illustrated in FIG. 11(A)and the transistor illustrated in FIG. 11(C) are calculated to evaluateelectrical characteristics of the transistors.

Here, as an index of the electrical characteristics of a transistor, theamount of change (hereinafter also referred to as ΔVsh) in the thresholdvoltage (hereinafter also referred to as Vsh) of the transistor is used.Note that in the Id-Vg characteristics, Vsh is defined as the value ofVg when Id=1.0×10⁻¹² [A] is satisfied.

Note that the Id-Vg characteristics are characteristics of a change in acurrent between the source and the drain (hereinafter also referred toas a drain current (Id)) when a potential applied to the conductor TGEfunctioning as a first gate of the transistor (hereinafter also referredto as a gate potential (Vg)) is changed from a first value to a secondvalue.

Here, evaluated are changes in a drain current (Id) when a potentialbetween a source and a drain (hereinafter also referred to as a drainpotential Vd) is set to +0.1 V and a potential between the source andthe conductor TGE functioning as a first gate is changed from −1 V to +4V.

A device simulator “Atlas” developed by Silvaco Inc. is used for thecalculation. Table 1 lists parameters used for the calculation. Notethat Eg represents an energy gap, Nc represents the effective density ofstates in the conduction band, and Nv represents the effective densityof states in the valence band.

TABLE 1 Software Atlas 2D produced by Silvaco Inc. Structure Channellength (L) 350 nm Channel width (W) 350 nm BGE Work function 5.0 eV Filmthickness (wiring) 20 nm Length in L direction 510 nm BGI Third layerRelative permittivity 4.1 Film thickness 10 nm Second layer Relativepermittivity 16.4 Film thickness 10 nm First layer Relative permittivity4.1 Film thickness 10 nm OS Electron affinity 4.5 eV Eg 2.9 eV Relativepermittivity 15 Electron mobility 20 cm²/Vs Hole mobility 0.01 cm²/Vs Nc5E+18 cm⁻³ Nv 5E+18 cm⁻³ Film thickness 15 nm n⁺ region Length in Ldirection 655 or 700 nm Carrier concentration 5E+18 cm⁻³ n⁻ regionLength in L direction 45 or 0  nm Carrier concentration 1E+17 cm⁻³ TGIRelative permittivity 4.1 Film thickness 10 nm SW Relative permittivity4.1 Width 15 nm TGE Work function 5.0 eV Film thickness 20 nm Length inL direction 350 nm S/D electrode Work function 4.5 eV

In the transistor illustrated in FIG. 11(A), one of the n⁺ regions isset to 700 nm, and one of then regions is set to 0 nm. In the transistorillustrated in FIG. 11(C), one of the n⁺ regions is set to 655 nm andone of the n regions is set to 45 nm. Each of the transistor illustratedin FIG. 11(A) and the transistor illustrated in FIG. 11(C) has astructure in which the second gate is larger than the i region. Notethat in this evaluation, a potential of the conductor BGE functioning asa second gate (hereinafter also referred to as a backgate potential(Vbg)) is set to 0.00 V, −3.00 V, or −6.00 V.

FIG. 11(B) shows the results of the Id-Vg characteristics obtained bythe calculation of the transistor illustrated in FIG. 11(A). The amountof change in the threshold voltage (ΔVsh) of the transistor when thebackgate potential is set to −3.00 V is +1.2 V as compared with thatwhen the backgate potential is set to 0.00 V. The amount of change inthe threshold voltage (ΔVsh) of the transistor when the backgatepotential is set to −6.00 V is +2.3 V as compared with that when thebackgate potential is set to 0.00 V. That is, the amount of change inthe threshold voltage (ΔVsh) of the transistor when the backgatepotential is set to −6.00 V is +1.1 V as compared with that when thebackgate potential is set to −3.00 V. Therefore, even when the potentialof the conductor BGE functioning as a second gate is made higher, theamount of change in the threshold voltage of the transistor is hardlychanged. In addition, even when the backgate potential is increased, therising characteristics are not changed.

FIG. 11(D) shows the results of the Id-Vg characteristics obtained bythe calculation of the transistor illustrated in FIG. 11(C). The amountof change in the threshold voltage (ΔVsh) of the transistor when thebackgate potential is set to −3.00 V is +1.2 V as compared with thatwhen the backgate potential is set to 0.00 V. The amount of change inthe threshold voltage (ΔVsh) of the transistor when the backgatepotential is set to −6.00 V is +3.5 V as compared with that when thebackgate potential is set to 0.00 V. That is, the amount of change inthe threshold voltage (ΔVsh) of the transistor when the backgatepotential is set to −6.00 V is +2.3 V as compared with that when thebackgate potential is set to −3.00 V. Therefore, the higher thepotential of the conductor BGE functioning as a second gate is made, thelarger the amount of change in the threshold voltage of the transistorbecomes. As the backgate potential is increased, the risingcharacteristics become worse.

As described above, it is found that in the transistor illustrated inFIG. 11(C), the higher the potential of the conductor BGE functioning asa second gate is made, the larger the amount of change in the thresholdvoltage of the transistor becomes. In contrast, in the transistorillustrated in FIG. 11(A), the amount of change in the threshold voltageof the transistor is not changed even when the potential of theconductor BGE functioning as a second gate is increased.

Note that this embodiment can be implemented in combination with theother embodiments described in this specification as appropriate.

Embodiment 3

In this embodiment, the composition of a metal oxide that can be used inthe OS transistor described in the above embodiment, will be described.

<Composition of Metal Oxide>

In this specification and the like, “CAAC (c-axis aligned crystal)” or“CAC (Cloud-Aligned Composite)” might be stated. Note that CAAC refersto an example of a crystal structure, and CAC refers to an example of afunction or a material composition.

A CAC-OS or a CAC-metal oxide has a conducting function in a part of thematerial and has an insulating function in a part of the material, andhas a function of a semiconductor as the whole material. Note that inthe case where the CAC-OS or the CAC-metal oxide is used in a channelformation region of a transistor, the conducting function is a functionof allowing electrons (or holes) serving as carriers to flow, and theinsulating function is a function of not allowing electrons serving ascarriers to flow. By the complementary action of the conducting functionand the insulating function, the CAC-OS or the CAC-metal oxide can havea switching function (On/Off function). In the CAC-OS or the CAC-metaloxide, separation of the functions can maximize each function.

In addition, the CAC-OS or the CAC-metal oxide includes conductiveregions and insulating regions. The conductive regions have theabove-described conducting function, and the insulating regions have theabove-described insulating function. In some cases, the conductiveregions and the insulating regions in the material are separated at thenanoparticle level. In some cases, the conductive regions and theinsulating regions are unevenly distributed in the material. Theconductive regions are observed to be coupled in a cloud-like mannerwith their boundaries blurred, in some cases.

Furthermore, in the CAC-OS or the CAC-metal oxide, the conductiveregions and the insulating regions each have a size of greater than orequal to 0.5 nm and less than or equal to 10 nm, preferably greater thanor equal to 0.5 nm and less than or equal to 3 nm and are dispersed inthe material, in some cases.

The CAC-OS or the CAC-metal oxide is formed of components havingdifferent bandgaps. For example, the CAC-OS or the CAC-metal oxide isformed of a component having a wide gap due to the insulating region anda component having a narrow gap due to the conductive region. Whencarriers flow in such a structure, carriers mainly flow in the componenthaving a narrow gap. The component having a narrow gap complements thecomponent having a wide gap, and carriers also flow in the componenthaving a wide gap in conjunction with the component having a narrow gap.Therefore, in the case where the above-described CAC-OS or CAC-metaloxide is used in a channel formation region of a transistor, highcurrent drive capability in the on state of the transistor, that is,high on-state current and high field-effect mobility, can be obtained.

In other words, the CAC-OS or the CAC-metal oxide can also be called amatrix composite or a metal matrix composite.

<Structure of Metal Oxide>

An oxide semiconductor is classified into a single crystal oxidesemiconductor and a non-single crystal oxide semiconductor. Examples ofa non-single crystal oxide semiconductor include a CAAC-OS (c-axisaligned crystalline oxide semiconductor), a polycrystalline oxidesemiconductor, an nc-OS (nanocrystalline oxide semiconductor), anamorphous-like oxide semiconductor (a-like OS), and an amorphous oxidesemiconductor.

Note that as a metal semiconductor used for a semiconductor of atransistor, a thin film having high crystallinity is preferably used.With the use of the thin film, the stability or the reliability of thetransistor can be improved. Examples of the thin film include a thinfilm of a single-crystal oxide semiconductor and a thin film of apolycrystalline oxide semiconductor. However, to form the thin film of asingle-crystal oxide semiconductor or the thin film of a polycrystallineoxide semiconductor over a substrate, a high-temperature process or alaser heating process is needed. Thus, the manufacturing cost isincreased, and in addition, the throughput is decreased.

Non-Patent Document 2 and Non-Patent Document 3 have reported that anIn—Ga—Zn oxide having a CAAC structure (referred to as CAAC-IGZO) wasfound in 2009. It has been reported that CAAC-IGZO has c-axis alignment,a crystal grain boundary is not clearly observed in CAAC-IGZO, andCAAC-IGZO can be formed over a substrate at low temperatures. It hasalso been reported that a transistor using CAAC-IGZO has excellentelectrical characteristics and high reliability.

In addition, in 2013, an In—Ga—Zn oxide having an nc structure (referredto as nc-IGZO) was found (see Non-Patent Document 4). It has beenreported that nc-IGZO has periodic atomic arrangement in a microscopicregion (for example, a region with a size greater than or equal to 1 nmand less than or equal to 3 nm) and there is no regularity of crystalorientation between different regions.

Non-Patent Document 5 and Non-Patent Document 6 have shown a change inaverage crystal size due to electron beam irradiation to thin films ofthe above CAAC-IGZO, the above nc-IGZO, and IGZO having lowcrystallinity. In the thin film of IGZO having low crystallinity,crystalline IGZO with a crystal size of approximately 1 nm was observedeven before the electron beam irradiation. Thus, it has been reportedthat the existence of a completely amorphous structure was not observedin IGZO. In addition, it has been shown that the thin film of CAAC-IGZOand the thin film of nc-IGZO each have higher stability to electron beamirradiation than the thin film of IGZO having low crystallinity. Thus,the thin film of CAAC-IGZO or the thin film of nc-IGZO is preferablyused for a semiconductor of a transistor.

The CAAC-OS has c-axis alignment, a plurality of nanocrystals areconnected in the a-b plane direction, and its crystal structure hasdistortion. Note that distortion refers to a portion where the directionof a lattice arrangement changes between a region with a uniform latticearrangement and another region with a uniform lattice arrangement in aregion where the plurality of nanocrystals are connected.

The nanocrystal is basically a hexagon but is not always a regularhexagon and is a non-regular hexagon in some cases. Furthermore, apentagonal lattice arrangement, a heptagonal lattice arrangement, andthe like are included in the distortion in some cases. Note that a clearcrystal grain boundary (also referred to as grain boundary) cannot beobserved even in the vicinity of distortion in the CAAC-OS. That is,formation of a crystal grain boundary is inhibited by the distortion ofa lattice arrangement. This is probably because the CAAC-OS can toleratedistortion owing to a low density of oxygen atom arrangement in an a-bplane direction, a change in interatomic bond distance by replacement ofa metal element, and the like.

Furthermore, the CAAC-OS tends to have a layered crystal structure (alsoreferred to as a layered structure) in which a layer containing indiumand oxygen (hereinafter, In layer) and a layer containing the element M,zinc, and oxygen (hereinafter, (M, Zn) layer) are stacked. Note thatindium and the element M can be replaced with each other, and when theelement M of the (M, Zn) layer is replaced by indium, the layer can alsobe referred to as an (In, M, Zn) layer. Furthermore, when indium of theIn layer is replaced by the element M, the layer can also be referred toas an (In, M) layer.

The CAAC-OS is an oxide semiconductor with high crystallinity. Bycontrast, in the CAAC-OS, it can be said that a reduction in electronmobility due to the crystal grain boundary is less likely to occurbecause a clear crystal grain boundary cannot be observed. Moreover,since the crystallinity of an oxide semiconductor might be decreased byentry of impurities, formation of defects, or the like, the CAAC-OS canbe regarded as an oxide semiconductor that has small amounts ofimpurities and defects (oxygen vacancies or the like). Thus, an oxidesemiconductor including a CAAC-OS is physically stable. Therefore, theoxide semiconductor including a CAAC-OS is resistant to heat and hashigh reliability. In addition, the CAAC-OS is stable with respect tohigh temperature in the manufacturing process (what is called thermalbudget). Accordingly, the use of the CAAC-OS for the OS transistor canextend a degree of freedom of the manufacturing process.

The nc-OS has a periodic atomic arrangement in a microscopic region (forexample, a region with a size greater than or equal to 1 nm and lessthan or equal to 10 nm, in particular, a region with a size greater thanor equal to 1 nm and less than or equal to 3 nm). In addition, noregularity of crystal orientation is observed between differentnanocrystals in the nc-OS. Thus, the orientation is not observed in thewhole film. Accordingly, in some cases, the nc-OS cannot bedistinguished from an a-like OS or an amorphous oxide semiconductordepending on an analysis method.

The a-like OS is an oxide semiconductor that has a structure betweenthose of the nc-OS and the amorphous oxide semiconductor. The a-like OShas a void or a low-density region. That is, the a-like OS has lowcrystallinity as compared with the nc-OS and the CAAC-OS.

An oxide semiconductor has various structures with different properties.Two or more kinds among the amorphous oxide semiconductor, thepolycrystalline oxide semiconductor, the a-like OS, the nc-OS, and theCAAC-OS may be included in an oxide semiconductor of one embodiment ofthe present invention.

<Transistor Including Oxide Semiconductor>

Next, the case where the above oxide semiconductor is used for atransistor will be described.

Note that when the above oxide semiconductor is used for a transistor, atransistor with high field-effect mobility can be achieved. In addition,a transistor with high reliability can be achieved.

Non-Patent Document 7 shows that the transistor using an oxidesemiconductor has an extremely low leakage current in an off state;specifically, the off-state current per micrometer in the channel widthof the transistor is of the order of yA/μm (10⁻²⁴ A/μm). For example, alow-power-consumption CPU utilizing a characteristic of low leakagecurrent of the transistor using an oxide semiconductor is disclosed (seeNon-Patent Document 8).

Furthermore, application of a transistor using an oxide semiconductor toa display device that utilizes the characteristic of a low leakagecurrent of the transistor has been reported (see Non-Patent Document 9).In the display device, a displayed image is changed several tens oftimes per second. The number of times an image is changed per second isreferred to as a refresh rate. The refresh rate is also referred to asdriving frequency. Such high-speed screen change that is hard for humaneyes to recognize is considered as a cause of eyestrain. Thus, it isproposed that the refresh rate of the display device is lowered toreduce the number of times of image rewriting. Moreover, driving with alowered refresh rate enables the power consumption of the display deviceto be reduced. Such a driving method is referred to as idling stop (IDS)driving.

An oxide semiconductor with a low carrier density is preferably used fora transistor. In the case where the carrier density of an oxidesemiconductor film is lowered, the impurity concentration in the oxidesemiconductor film is lowered to lower the density of defect states. Inthis specification and the like, a state with a low impurityconcentration and a low density of defect states is referred to as ahighly purified intrinsic or substantially highly purified intrinsicstate. For example, the carrier density of the oxide semiconductor islower than 8×10¹¹/cm³, preferably lower than 1×10¹¹/cm³, more preferablylower than 1×10¹⁰/cm³, and greater than or equal to 1×10⁻⁹/cm³.

In addition, a highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has a low density of defectstates and accordingly has a low density of trap states in some cases.

Furthermore, charges trapped by the trap states in the oxidesemiconductor take a long time to disappear and may behave like fixedcharges. Thus, a transistor whose channel region is formed in an oxidesemiconductor with a high density of trap states has unstable electricalcharacteristics in some cases.

Thus, in order to stabilize electrical characteristics of thetransistor, reducing the impurity concentration in the oxidesemiconductor is effective. Furthermore, in order to reduce the impurityconcentration in the oxide semiconductor, it is preferred that theimpurity concentration in an adjacent film be also reduced. Examples ofimpurities include hydrogen, nitrogen, an alkali metal, an alkalineearth metal, iron, nickel, and silicon.

<Impurity>

Here, the influence of each impurity in the oxide semiconductor will bedescribed.

When silicon or carbon, which is one of the Group 14 elements, iscontained in the oxide semiconductor, defect states are formed in theoxide semiconductor. Thus, the concentration of silicon or carbon in theoxide semiconductor and the concentration of silicon or carbon in thevicinity of an interface with the oxide semiconductor (the concentrationobtained by secondary ion mass spectrometry (SIMS)) are set lower thanor equal to 2×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁷atoms/cm³.

Furthermore, when the oxide semiconductor contains an alkali metal or analkaline earth metal, defect states are formed and carriers aregenerated in some cases. Thus, a transistor using an oxide semiconductorthat contains an alkali metal or an alkaline earth metal is likely tohave normally-on characteristics. Accordingly, it is preferred to reducethe concentration of an alkali metal or an alkaline earth metal in theoxide semiconductor. Specifically, the concentration of an alkali metalor an alkaline earth metal in the oxide semiconductor obtained by SIMSis set lower than or equal to 1×10¹⁸ atoms/cm³, preferably lower than orequal to 2×10¹⁶ atoms/cm³.

Furthermore, when containing nitrogen, the oxide semiconductor easilybecomes n-type by generation of electrons serving as carriers and anincrease of carrier density. As a result, a transistor using an oxidesemiconductor containing nitrogen as a semiconductor is likely to havenormally-on characteristics. For this reason, nitrogen in the oxidesemiconductor is preferably reduced as much as possible; the nitrogenconcentration in the oxide semiconductor obtained by SIMS is set, forexample, lower than 5×10¹⁹ atoms/cm³, preferably lower than or equal to5×10¹⁸ atoms/cm³, more preferably lower than or equal to 1×10¹⁸atoms/cm³, and still more preferably lower than or equal to 5×10¹⁷atoms/cm³.

Furthermore, hydrogen contained in the oxide semiconductor reacts withoxygen bonded to a metal atom to be water, and thus forms an oxygenvacancy in some cases. Entry of hydrogen into the oxygen vacancygenerates an electron serving as a carrier in some cases. Furthermore,in some cases, bonding of part of hydrogen to oxygen bonded to a metalatom causes generation of an electron serving as a carrier. Thus, atransistor using an oxide semiconductor containing hydrogen is likely tohave normally-on characteristics. Accordingly, hydrogen in the oxidesemiconductor is preferably reduced as much as possible. Specifically,the hydrogen concentration in the oxide semiconductor obtained by SIMSis lower than 1×10²⁰ atoms/cm³, preferably lower than 1×10¹⁹ atoms/cm³,more preferably lower than 5×10¹⁸ atoms/cm³, and still more preferablylower than 1×10¹⁸ atoms/cm³.

When an oxide semiconductor with sufficiently reduced impurities is usedfor a channel formation region of a transistor, stable electricalcharacteristics can be given.

The discovery of the CAAC structure and the nc structure has contributedto an improvement in electrical characteristics and reliability of atransistor using an oxide semiconductor having the CAAC structure or thenc structure, a reduction in manufacturing cost, and an improvement inthroughput. Furthermore, applications of the transistor to a displaydevice and an LSI utilizing the characteristics of a low leakage currentof the transistor have been studied.

Note that this embodiment can be implemented in combination with theother embodiments described in this specification as appropriate.

Embodiment 4

In this embodiment, examples of electronic devices each including theimaging device 10 described in the above embodiment will be described.

FIG. 12(A) illustrates a surveillance camera which includes a housing951, a lens 952, a support portion 953, and the like. The surveillancecamera can include the imaging device of one embodiment of the presentinvention as a component for obtaining an image. Thus, the surveillancecamera can be reduced in size and weight. Furthermore, a highly reliablesurveillance camera can be provided even in a high temperatureenvironment. Note that a surveillance camera is a name in common use andthe name does not limit the use thereof. A device that has a function asa surveillance camera is also referred to as a camera or a video camera,for example.

FIG. 12(B) illustrates a video camera which includes a first housing971, a second housing 972, a display portion 973, operation buttons 974,a lens 975, a connection portion 976, and the like. The operation button974 and the lens 975 are provided on the first housing 971, and thedisplay portion 973 is provided on the second housing 972. The videocamera can include the imaging device of one embodiment of the presentinvention as a component for obtaining an image. Thus, the video cameracan be reduced in size and weight. Furthermore, a video camera capableof performing imaging for a long time with low power consumption can beprovided. A video camera capable of high-speed imaging can be provided.

FIG. 12(C) illustrates a digital camera which includes a housing 961, ashutter button 962, a microphone 963, a light-emitting portion 967, alens 965, and the like. The digital camera can include the imagingdevice of one embodiment of the present invention as a component forobtaining an image. Thus, the digital camera can be reduced in size andweight. Furthermore, a digital camera capable of high-speed imaging canbe provided.

FIG. 12(D) illustrates a mobile phone (smartphone) which includes adisplay portion 982, a microphone 987, a speaker 984, a camera 989, aninput/output terminal 986, an operation button 985, and the like in ahousing 981. A touch panel function of the display portion 982 enablesinput and output of information. The mobile phone can include theimaging device of one embodiment of the present invention as a componentfor obtaining an image. Thus, the mobile phone can be reduced in sizeand weight. Furthermore, a mobile phone capable of high-speed imagingcan be provided. A mobile phone in which power consumption of an imagingoperation is reduced can be provided.

A robot 900 illustrated in FIG. 13 includes an arithmetic device 910, anilluminance sensor 901, a microphone 902, an upper camera 903, a speaker904, a display 905, a lower camera 906, an obstacle sensor 907, and amoving mechanism 908. The upper camera 903 and the lower camera 906 eachhave a function of taking an image of the surroundings of the robot 900.The obstacle sensor 907 can detect, with the use of the moving mechanism908, the presence of an obstacle in the direction where the robot 900advances. The robot 900 can move safely by recognizing the surroundingswith the upper camera 903, the lower camera 906, and the obstacle sensor907.

In the robot 900, the upper camera 903 and the lower camera 906 can eachinclude the imaging device of one embodiment of the present invention asa component for obtaining an image. Thus, the robot can be reduced insize and weight. Furthermore, a highly reliable robot capable oftransferring safely even in a high temperature environment can beprovided.

Note that the electronic devices, the functions of the electronicdevices, its effects, and the like described in this embodiment can becombined as appropriate with the description of another electronicdevice. This embodiment can be implemented in combination with the otherembodiments described in this specification as appropriate.

Example

In this example, a DRAM (DOSRAM) in which a transistor using an oxidesemiconductor is used for a memory cell was fabricated, and it wasconfirmed that the fabricated DOSRAM can retain the stored content for100000 seconds. Furthermore, it was estimated that a reduction inrefresh rate can achieve a 50% reduction in power consumption at maximumas compared with a conventional DRAM.

Table 2 shows the specifications of the fabricated DOSRAM. In theDOSRAM, a 60-nm OSFET process is used for the memory cell and an oxidesemiconductor transistor is used. For a driver circuit that has afunction of selecting a memory cell, writing data to a memory cell,reading out data stored in a memory cell, or the like, a 65-nm CMOSprocess is employed and a Si transistor is used.

TABLE 2 Technology CMOS 65 nm OSFET 60 nm Supply Voltage VDD: 1.2 V/VPP:3.3 V Cell element 1Tr1C Cs value 3.5 fF Density 64 kbit Cycle time 10ns Retention time 100,000 sec. I/O width ×32

FIG. 14(A) shows a Shmoo plot at 25° C. and FIG. 14(B) shows a Shmooplot at 85° C. It is found that operation with a cycle time of 10 ns canbe performed under conditions that the supply voltage is 1.2 V and thetemperature range is from 25° C. to 85° C. This satisfies thespecifications of the conventional DRAM; accordingly, the fabricatedDRAM can probably maintain compatibility with the conventional DRAM.

FIG. 15 shows memory retention characteristics (Retentioncharacteristics) at 85° C. Favorable data retention characteristics witha rate of correct bit of 99.97% are shown after 10⁵ seconds.

FIG. 16 shows the estimation results of the power consumption (PowerConsumption Ratio) in the case where power gating is performed in aDRAM, a DOSRAM, and a DOSRAM. In FIG. 16 , it is assumed that the oxidesemiconductor transistor can be miniaturized and the same cell size andmemory capacitance as a DRAM can be achieved. FIG. 16(A) shows an I/Onumber of x4 and memory capacity of 4 Gbit, FIG. 16(B) shows an I/Onumber of x8 and memory capacity of 8 Gbit, and FIG. 16(C) shows an I/Onumber of x16 and memory capacitance of 16 Gbit.

Since refresh operation is unnecessary for a DOSRAM, power needed forthe refresh operation can be saved. Furthermore, power gating can reducestandby power. The proportion of the power required for the refreshoperation increases as the memory density is higher and the I/O numberis smaller. As a result, it was estimated that the total power can bereduced by 50% at maximum (see FIG. 16(A)). Note that for thecalculation, “Micron DDR4 SDRAM System-Power Calculator” produced byMicron Technology, Inc. was used.

REFERENCE NUMERALS

-   a1: node, a2: node, a3: node, a4: node, N1: node, S: oxide, S1:    oxide, SUB1: semiconductor substrate, SUB2: semiconductor substrate,    10: imaging device, 15: imaging device, 20: light, 100: layer, 110:    signal processing circuit, 111: control circuit, 112: control    circuit, 113: image processing circuit, 114: input/output circuit,    115: bus line, 116: power supply circuit, 200: layer, 200 a: layer,    201: conductor, 210: memory device, 211: cell array, 212: memory    cell, 213: transistor, 214: capacitor, 215: transistor, 221: driver    circuit, 222: driver circuit, 300: layer, 301: conductor, 310: image    sensor, 311: pixel array, 312: pixel, 313: photoelectric conversion    element, 314: transistor, 315: transistor, 316: transistor, 317:    transistor, 318: capacitor, 321: driver circuit, 322: driver    circuit, 331: node, 332: wiring, 333: wiring, 334: wiring, 400:    transistor, 400 a: transistor, 400 b: transistor, 411: semiconductor    substrate, 413: semiconductor region, 414 a: low-resistance region,    414 b: low-resistance region, 415: insulator, 416: conductor, 417:    semiconductor region, 418 a: low-resistance region, 418 b:    low-resistance region, 420: insulator, 422: insulator, 424:    insulator, 426: insulator, 428: conductor, 430: conductor, 450:    insulator, 452: insulator, 454: insulator, 456: conductor, 460:    insulator, 462: insulator, 464: insulator, 466: conductor, 500:    transistor, 500A: transistor, 503: conductor, 503 a: conductor, 503    b: conductor, 510: insulator, 512: insulator, 514: insulator, 516:    insulator, 518: conductor, 520: insulator, 522: insulator, 524:    insulator, 530: oxide, 530 a: oxide, 530 b: oxide, 530 c: oxide, 540    a: conductor, 540 b: conductor, 542: conductor, 542 a: conductor,    542 b: conductor, 543: region, 543 a: region, 543 b: region, 544:    insulator, 545: insulator, 546: conductor, 548: conductor, 550:    insulator, 560: conductor, 560 a: conductor, 560 b: conductor, 574:    insulator, 580: insulator, 581: insulator, 582: insulator, 586:    insulator, 600: capacitor, 610: conductor, 612: conductor, 620:    conductor, 630: insulator, 646: conductor, 648: conductor, 650:    insulator, 660: conductor, 700 a: transistor, 700 c: photodiode,    711: semiconductor substrate, 713: semiconductor region, 714 a:    low-resistance region, 714 b: low-resistance region, 715: insulator,    716: conductor, 718: p-type semiconductor region, 720: insulator,    722: insulator, 724: insulator, 726: insulator, 728: conductor, 730:    conductor, 750: insulator, 752: insulator, 754: insulator, 756:    conductor, 760: insulator, 762: insulator, 764: insulator, 766:    conductor, 770: conductor, 900: robot, 901: illuminance sensor, 902:    microphone, 903: upper camera, 904: speaker, 905: display, 906:    lower camera, 907: obstacle sensor, 908: moving mechanism, 910:    arithmetic device, 951: housing, 952: lens, 953: support portion,    961: housing, 962: shutter button, 963: microphone, 965: lens, 967:    light-emitting portion, 971: housing, 972: housing, 973: display    portion, 974: button, 975: lens, 976: connection portion, 981:    housing, 982: display portion, 984: speaker, 985: button, 986:    input/output terminal, 987: microphone, 989: camera.

1. A semiconductor device comprising: a first layer comprising aplurality of circuits; a second layer above the first layer, the secondlayer comprising a plurality of memory cells electrically connected tothe plurality of circuits; and a third layer above the second layer, thethird layer comprises an image sensor electrically connected to theplurality of circuits, wherein one of the plurality of circuitscomprises a first transistor, wherein the first transistor comprises afirst crystalline semiconductor material in a channel formation region,wherein each of the plurality of memory cells comprises a secondtransistor, wherein the second transistor comprises a metal oxide in achannel formation region and a gate and a back gate with the channelformation region provided therebetween, wherein the image sensorcomprises a plurality of pixels each comprising a third transistor and aphotodiode, wherein the third transistor comprises a second crystallinesemiconductor material in a channel formation region, and wherein asemiconductor region of the photodiode comprises the second crystallinesemiconductor material.
 2. The semiconductor device according to claim1, wherein at least one of the first crystalline semiconductor materialand the second crystalline semiconductor material is single crystalsilicon.
 3. The semiconductor device according to claim 1, wherein thechannel formation region of the first transistor is provided in a firstsemiconductor substrate, and wherein the channel formation region of thethird transistor is provided in a second semiconductor substrate.
 4. Thesemiconductor device according to claim 1, wherein the metal oxidecomprises at least indium and zinc.
 5. The semiconductor deviceaccording to claim 1, wherein the plurality of pixels are stacked overthe plurality of the memory cells.
 6. The semiconductor device accordingto claim 1, wherein at least part of the third transistor overlaps withthe second transistor and the first transistor.
 7. A semiconductordevice comprising: a first layer comprising a plurality of circuits; asecond layer above the first layer, the second layer comprising aplurality of memory cells electrically connected to the plurality ofcircuits; and a third layer above the second layer, the third layercomprises an image sensor electrically connected to the plurality ofcircuits, wherein one of the plurality of circuits comprises a firsttransistor, wherein the first transistor comprises a first crystallinesemiconductor material in a channel formation region, wherein each ofthe plurality of memory cells comprises a second transistor and acapacitor, wherein the second transistor comprises a metal oxide in achannel formation region, a source, a drain, and a gate and a back gatewith the channel formation region provided therebetween, wherein a firstelectrode of the capacitor is electrically connected to one of thesource and the drain of the second transistor, wherein the image sensorcomprises a plurality of pixels each comprising a third transistor and aphotodiode, wherein the third transistor comprises a second crystallinesemiconductor material in a channel formation region, and wherein asemiconductor region of the photodiode comprises the second crystallinesemiconductor material.
 8. The semiconductor device according to claim7, wherein at least one of the first crystalline semiconductor materialand the second crystalline semiconductor material is single crystalsilicon.
 9. The semiconductor device according to claim 7, wherein thechannel formation region of the first transistor is provided in a firstsemiconductor substrate, and wherein the channel formation region of thethird transistor is provided in a second semiconductor substrate. 10.The semiconductor device according to claim 7, wherein the metal oxidecomprises at least indium and zinc.
 11. The semiconductor deviceaccording to claim 7, wherein the plurality of pixels are stacked overthe plurality of the memory cells.
 12. The semiconductor deviceaccording to claim 7, wherein at least part of the third transistoroverlaps with the second transistor, the capacitor, and the firsttransistor.